"In fact, why do they allow these wierd cults to exist anyway if they harm people or break the law?"
Because under American law, at least, being "weird' or preaching "crazy" doctrines is not in itself a charge under the law. If Sam Citizen wants to start a cult teaching that he has five heads, but that only the pure of heart can see more than one, he is perfectly free to do so.
Besides, if we outlaw "weird cults" there will be many diverse individuals demanding that Roman Catholics and/or Lutherans and/or Methodists and/or Baptists and/or Quakers and/or Jews and/or Muslims be prosecuted as exactly that, "weird cults." One person's "weird" is another person's "sacred."
One of the problems is that by a time a cult leader finds himself (herself) surrounded by adoring acolytes he (she) starts viewing himself (herself) as ABOVE the law. And it apparently doesn't take all that many followers. (David Koresh's entourage was never all that large.)
But you can't prosecute people for what they MAY do in the future.
If that was the case, you could imprison everybody short on cash as "potential bank robbers."