Many evangelicals regard Mormonism as nothing more than a cult: and a cult, moreover, that is based not only on a false theology but also on a willingness to tamper with the inerrant word of God that is the Bible.
You may be completely right in your interpretation of the Christian Bible but America is not a theocracy and neither the Legislature nor the Courts can base their legislation or their rulings on stories from the Bible anymore than they can base them on the various books that other religions contend state the holy and inerrant truth.