Actually it was the Supreme Court themselves who made the ridiculous leap from a typical 1st amendment interpretation of religious freedom to human sacrifice, and they did it in the case I cited. They explicitly used that example to say that the first amendment couldn't possibly protect religious action, only religious thought, because religious action could be taken too far. So as far as freedom of religion is concerned, yeah they basically nullified first amendment protections. In practice that hasn't happened much, but the point is there's a well known Supreme Court precedent they can point to any time they want.
The case wasn't even about human sacrifice. It was about consenting adults considering themselves married in a religious sense, and sharing living space, finances, relationships, etc. I get the argument that as long as there are tax benefits to a marriage, multiplying the benefit through polygamy (or any marriage not sanctioned by the state) could be seen as fraud. But as far as I'm aware no one was trying to do anything of the sort. The arrests directly and indirectly related to this case were all predicated on who was associating with who and in what way, and the Supreme Court upheld the decision that religion was not a defense for any of that.
It's about as ridiculous as someone saying that no one can have a homosexual marriage and sodomy should be a criminal offense just because it violates that one person's religion.
edit: Yeah I looked it up and my memory serves me correctly. They ruled against the religious freedom argument based on what I said above, and the fact that laws against bigamy existed from the time of a previous King of England. Which is silly since the puritans came here seeking religious freedom from... the King of England.
The Court recognized that under the First Amendment, the Congress cannot pass a law that prohibits the free exercise of religion. However it held that the law prohibiting bigamy did not meet that standard. The principle that a person could only be married singly, not plurally, existed since the times of King James I of England in English law, upon which United States law was based. The Court investigated the history of religious freedom in the United States and quoted a letter from Thomas Jefferson in which he wrote that there was a distinction between religious belief and action that flowed from religious belief. The former "lies solely between man and his God," therefore "the legislative powers of the government reach actions only, and not opinions." The court considered that if polygamy was allowed, someone might eventually argue that human sacrifice was a necessary part of their religion, and "to permit this would be to make the professed doctrines of religious belief superior to the law of the land, and in effect to permit every citizen to become a law unto himself." The Court believed the First Amendment forbade Congress from legislating against opinion, but allowed it to legislate against action. -- Wikipedia article on Reynolds v. United States
So yeah you're free to believe or think whatever you want. But when it comes to worshiping according to the dictates of your own conscience, for some reason the seemingly obvious line of "as long as you're not harming anyone" or "as long as you're not taking away anyone else's freedom" didn't come up.