According to HG.org, a website dedicated to giving out legal advice and helping people find attorneys, declaring a cult illegal is a tricky thing. Because many “cults” operate under the guise of being religious groups, they are protected by the First Amendment’s freedom of religious exercise. The kicker is, if these cults hinder the freedom of their members in any way, then they can be dismantled for criminal activity.
HG.org also points out, however, that religious cult leaders have the ability to change objectives and activities at the drop of a hat.
According to the site, “Leaders can often change the tenets of the religion on a whim, making it difficult to impossible for lawmakers to get ahead of potentially dangerous groups or to identify standards by which a cult becomes illegal. Thus, most groups that would be considered illegal have already engaged in criminal conduct.”