One has to wonder why the States have turned a blind eye on it for so long, and apparently still are in some cases.
This gives a good history: http://www.childbrides.org/history.html
There is a whole list of reasons why they have turned an eye and do not go after these groups. I really encourage people to read "Under the Banner of Heaven"; it outlines the entire culture from the beginning to present. The US turned its head up until the 1950's. During the 50's the feds put pressure on the state of Utah to do something about it. That time period is now known as "the Raids". State and fed agencies raided these compounds to break them apart. All it did was drive them further underground. Some groups located to Mexico and Canada where the US cannot touch them.
There is also some big politics involved in the issues too. The original belief of the Mormons is that polygamy is OK. It's one of the original principles. Because Mormonism is a recognized religion in the US, many of the polygamist groups have argued in court, for example, that polygamy is a religious practice and should be out of government control. It's such a hot button issue in Utah, AZ and NM that many have not wanted to touch it. Most states look away as long as adults are involved but the public outcry became to deafening when it was learned the Jeffs group was hurting young kids. The feds had no choice but to act... finally.
One reason why the US took until the 1950's to act was they needed Utah. Utah was the connecting point when this country was developing. Traders and those headed west stopped in Utah to trade and many of the first routes west went through Utah. If the fed government wanted to keep developing the country and using Utah to pass through they had to stay out of Utah politics. That became the status quo and the fed government left Utah alone. In the 1950's public outcry popped up and "the Raids" took place. It seems like they will never get rid of the practice, just "deal with it" from time to time. (Read the book! Really!) The Short Creek was the most famous raid: http://www.childbrides.org/history_scraid1.html
There are some wild ideas with in the Mormon religion. The Golden Mountain always gets me. The thought is that when Judgment day comes, those that believe it - will hide within the Mountain (it's an actual mt in Utah). They built a mineshaft with rooms etc; the mountain is basically a bomb shelter. Well, folks can hide in it and once God has finished wiping the Earth clean, people can emerge from the Mountain to restart the planet. They also believe that God will fill the Mountain with gold so they will have money to get back up and running. I want a Mountain with gold in it!
H.R.H. Nicole Marie
Eve was Framed