A lot of people are raised in religions that are very misogynistic. My husband's family is Southern Baptist. That pastor tried to feed me that men are superior song-and-dance and include it in the vows, too. That doesn't mean that the women in his congregation don't have free will. I think it's sad, but a lot of the women genuinely believe that's how God wants them to live their lives. What if you were one of those young girls, say a 16-year-old pregnant wife, who was forced out of that lifestyle and into our lifestyle, which I'm sure they find as wrong as we find theirs? There's no free will in *that* either. I, for one, would be angry and appalled. Even if those girls have been brainwashed, those are their religious beliefs. They are important to those girls and they have a right to them. Who are we to forcibly rip them off a path that they believe God ordered?
Most organized religions brainwash to some extent. They all pretty much say these are the things you *must* do, believe, and/or say in order to be a good person, to earn the love of God/the Gods, or to reach some sort of spiritual reward. Many of those religions predate feminism and are, therefore, very sexist. Some are catching up. Others are becoming further entrenched. The bottom line is that we can't make feminism mandatory without infringing upon the rights of numerous religious groups, not just these polygamists.
We've done what we can do. We can give women equal rights under the law, which we've done. We can open doors, which we've done. We can expand health care and medical knowledge so that women aren't being treated according to the typical male model (we are slowly starting to do this, but I really wish we'd move a bit faster). What we can't do is force people to change their lifestyles against their wills and against their religions. It is troubling, but many people -- many women, even -- just don't see women as equal to men. In a free nation, those people should be allowed to deviate from the mainstream as much as they choose, provided they don't commit any crimes. If the police can find evidence of actual crimes, then sure. Make arrests. Take away the underage victims of those specific crimes. But you can't say that because they don't embrace feminism, the women have no free will and cannot consent, and so everything they did is a crime. There is no law saying these people have to be feminists. If there were such a law, a lot of churches would have to be shut down. I wouldn't want to live that way, but I wouldn't want the government to swoop down and tell me my religion is illegal, either.
With regard to anti-polygamy laws... I think those were put in place to entice more women to come and/or stay West. The West had earned itself a pretty wild reputation. On the one hand, women in some of the territories enjoyed the right to vote. On the other hand, women were viewed by the overwhelmingly male society as rare commodities and freely bought and sold (and not always by choice). Chinese girls were brought over in boatloads as prostitute slave girls. By and large, women were considered property. A father might lose his daughter in a game of cards. That was no way to entice families and women to come West, so I think a lot of communities tightened their polygamy and prostitution laws in consideration of the fact that if they were going to build a real and lasting society that would be around after the gold and other resources were gone, they had to bill themselves as less “wild” and more civilized and safe. Now that women’s equal rights are protected by the law, though, those anti-polygamy and anti-prostitution laws only serve to promote discrimination against women who choose to live as polygamists or prostitutes, respectively.