Government cannot correct all of the evils in a society. To think it should is a utopian concept. Utopian ideals are at the core of Communism, Nazism, and Islamic radicalism. Given the dismal history of governments in the last century, it is better to restrain state power than to use it indiscriminately, even when some good may be accomplished.
A polygamous cult such as the FDLS is immoral and abusive of its members, but the danger from unrestrained government power is greater, even when done with the best of intentions.