I agree with Kerry in being a bit perplexed by what seems to me unreflective anti-gubmint reactions of libertarians to the FLDS imbroglio. It seems clear enough to me that these kids are basically brainwashed, isolated, and made dependent in a way that makes it all-but-impossible for them to freely choose this way of life or ever to have the capacity to exercise their liberty in a meaningful way. Individuation and the minimal conditions for self-government don't develop all by themselves, but we each have legitimate moral and political claims against our parents for their development. The state should step in if parents violate their kids' basic rights, because protecting rights are what states are for.
I understand the slippery slope argument here. But this is child abuse and evangelical homeschooling isn't, and it's important to be able articulate the difference. If you can't figure out how to articulate the difference, then you don't infer that child abuse is OK. You infer that evangelical home-schooling is child abuse, too — so you'd better be able to articulate the difference. If the government has overstepped its legal powers in this particular case, then they've overstepped their legal powers. But that might just mean that it needs to be easier for the state to protect children against brainwashing and rape. Apologizing for it doesn't seem to me a coherently libertarian position.
The libertarian point is that the illegality and attendant marginalization of polygamy pushes it into isolated, authoritarian, quasi-state cult compounds where these kinds of crimes are most likely to take place.