Lots of reservations have large numbers of Episcopalians, for the same reason. And other branches of Christianity. "Native language = Native religion" is not going to be correct. I'll also add that Muturzikin is a beautiful website and a great source of info, but their maps can be misleading. They like to show minority languages as spots on the map, which can give the impression that a majority of speakers in that area speak the language. That's often not true; the spots sometimes just indicate the presence of a minority language in that area.