First off it is violation of domestic law. Religion in American is a consensual affair at all times not permanent contract. A person's relationship with a religion ends the moment they say it ends. It is a violation of first amendment rights to assert religious authority over someone without their consent. Marian Guinn vs Church of Christ Collinsville is an important case where the courts were definitive, "No real freedom to choose religion would exist in this land if under the shield of the First Amendment religious institutions could impose their will on the unwilling and claim immunity from secular judicature for their tortious acts." A similar case involving a Mormon was Norman Hancock, with the same result the Hancock was awarded damanges as the court saw continuing a disciplinary process on a non member to be a violation of their civil rights.
Permanent church covenants or statements that discipline will continue after a person tries to leave in no way alter any of the above. They are useful for establishing informed consent to starting a disciplinary process, and continuing it while someone remains a member. But, the law and the courts don't consider religious practice and affiliation to be a contract but rather a civil right. You can't sign away your right to quit a church any more than you can sign away your right to sue for sexual harassment in the workplace. Wollersheim v. Church of Scientology which was twice appealed all the way to the US Supreme Court was unequivocal in its finding that permanent consent cannot be granted. Consent to being disciplined, is like consent to being subjected to any other religious practice and can be revoked at will. For an older case, O'Moore v. Driscoll established that lack of consent to a ritual instantly removes privilege. In Scolinki the courts found that church discipline arises from common interest. A person voluntarily leaving the religious community has severed their common interest, which is why I frequently say that churches can excommunicate immediately or record what they want when a person leaves but not engage in an ongoing process.
Second it is a violation of international human rights law: Everyone has the right to freedom of thought, conscience and religion; this right includes freedom to change his religion or belief, and freedom, either alone or in community with others and in public or private, to manifest his religion or belief in teaching, practice, worship and observance. (Universal Declaration of Human Rights, Article 18).
1. Everyone has the right to freedom of conscience and of religion. This right includes freedom to maintain or to change one's religion or beliefs, and freedom to profess or disseminate one's religion or beliefs, either individually or together with others, in public or in private.2. No one shall be subject to restrictions that might impair his freedom to maintain or to change his religion or beliefs. (American Convention on Human Rights, Article 12)
Every man has a right to withdraw from the Church whenever he pleases, in the sense explained in our former article -- a right in the sense that no human authority has the right to detain him. As before God, he has no more right to apostatize than to commit any other sin. He is bound to believe and keep the commandments. But men have no commission to force him to do either. If he wants to go, they must let him go. "They went out from us," says the Apostle -- not that they were expelled, but they went out of their own accord, freely, voluntarily -- "because they were not of us." They found themselves in the wrong place, and they left it. (The Collected Writings of James Henley Thornwell, Vol.4: Ecclesiastical, p. 370.)To pick a Baptist quote:
Baptists have one consistent record concerning liberty throughout all their long and eventful history. They have never been a party to oppression of conscience. They have forever been the unwavering champions of liberty, both religious and civil. Their contention now, is, and has been, and, please God, must ever be, that it is the natural and fundamental and indefeasible right of every human being to worship God or not, according to the dictates of his conscience, and, as long as he does not infringe upon the rights of others, he is to be held accountable alone to God for all religious beliefs and practices. Our contention is not for mere toleration, but for absolute liberty. There is a wide difference between toleration and liberty. Toleration implies that somebody falsely claims the right to tolerate. Toleration is a concession, while liberty is a right. Toleration is a matter of expediency, while liberty is a matter of principle. Toleration is a gift from God. It is the consistent and insistent contention of our Baptist people, always and everywhere, that religion must be forever voluntary and uncoerced, and that it is not the prerogative of any power, whether civil or ecclesiastical, to compel men to conform to any religious creed or form of worship, or to pay taxes for the support of a religious organization to which they do not believe. God wants free worshipers and no other kind. (By George Truett, Southern Baptist Convention, May 16 1920).Among Presbyterians the notion of "erasure without consent of the session" or renouncement of jurisdiction is a well established right. For example the largest Presbyterian denomination, the PC(USA) is unequivocal that, "Members, church officers, elders and ministers have the right to renounce jurisdiction at any time. " To pick from the opposite end of the ideological spectrum the Orthodox Presbyterian Church considers leaving without permission of the session to be an erasure but not an excommunication, a termination of membership (Book of Discipline II.B.3.d.1,3,5).
To appreciate the breadth and unanimity of this I'll note that Jehovah's witnesses have a similar notion with different terminology. A member who is excommunicated is called, "disfellowshiped" while one who leaves on their own is "disassociated". There is a clear understanding that the Watchtower bible and tract society cannot claim disciplinary authority over a person who no longer considers themselves a member of the society (see wikipedia for more details).
And again to indicate the breadth, a person who declares themselves to no longer be a member of the church of the latter day saints is a "disaffiliate", and not subject to a disciplinary council (which can pronounce excommunication), "Nor are [Disciplinary Councils] held for members who demand that their names be removed from Church records or who have joined another church; that is now an administrative action." (from A Chance to Start Over: Church Disciplinary Councils and the Restoration of Blessings)
To summarize, it may be the case that the church views the status of one who voluntarily left as having excommunicated themselves, "erasure is an excommunication", or to quote Jim West, "The New Testament identifies two classes of excommunicates: first, those who have been formally sentenced by the Church of Christ (1 Corinthians 5:5). Second, those that excommunicate themselves by leaving the pale of the visible church in order to feast in the world (1 John 2:19)" (link). But the right to leave without engaging in an extended process, is guaranteed by church tradition.
Fourth, treating excommunication as more than simply leaving a church confuses excommunication and anathema. Excommunication is an involuntary separation of communion from a particular group of brethren. Anathema is a separation from God. If a person chooses to separate communion themselves then they are in some sense self excommunicating, by continuing an official process the church is asserting an authority over the status of their spirit, an authority they should not claim to possess. Pope John VIII makes this clear in the decree of Gratian (c. III, q. V, c. XII), "... excommunication, which separates her from the society of the brethren, but under the anathema, which separates from the body of Christ, which is the Church". There is an earlier article on this site which addresses the distinction in more detail.
Fifth, it is unbiblical. Almost every passage dealing with discipline is specific to a church. For example Rev 2:20 the condemnation for the false teacher is only against the church of Thyatira and not against the church universal. 1Cor 5:12, "For what do I have to do with judging those outside? Are you not to judge those inside?" gives churches the responsibility to judge those within the church and excludes those outside the church. So to argue that judgment should continue is to argue that one can be part of a church even though openly leaving it. But 1John 2:19 says the exact opposite, "They went out from us, but they did not really belong to us, because if they had belonged to us, they would have remained with us. But they went out from us to demonstrate that all of them do not belong to us."
Sixth, it is seen by the world as harassment and thus a negative witness. The cartoon I opened this with gives a good picture of how the world sees this. Churches enjoy substantial latitude in the United States because they are voluntary associations, that is everyone is a member because they have chosen to be a member and are in no way coerced into remaining members. Organizations that are mandatory or make it difficult to leave for example: places of employment, business partnerships, condominium associations are subjected to substantially higher levels of regulation and oversight. Society believes people need protections against organizations that are compulsory in a way they do not in organizations that are voluntary. Americans regardless of their affiliation believe strongly in the notion that religion is voluntary, religions that are seen in any way to move towards compulsory are thought of quite negatively and treated quite harshly.
To help Christians from disciplining communities see this the way the world sees it, lets use an analogy. Picture a person who at one time had joined a coven and agreed forever to allow themselves to be bled into a common chalice during circle rites. They quit the coven, does the coven have the right to continue to bleed them?