As an atheist, I have the same feeling about all religions (I'm okay with them, as everyone is free to believe whatever they want), so long as their adherents are not batshit insane. These fundamentalists believe that mainline Mormons sold the religion out by forsaking polygamy when Utah joined the United States. The book is not only about polygamy. It's about the physical, sexual, and child abuse perpetrated by the male fundamentalists on their plural wives and children. Many of the women they end up marrying are under 18 and are often their first cousins, nieces, or step-daughters. It's gross.
Colorado City, Arizona, is one such fundamentalist community. They are left alone because of their remoteness. It is in the northwest corner of the state of Arizona and it is completely cut off from the rest of the state by the Grand Canyon.