I actually haven't seen it, you're right, but my understanding is that it's about the Sioux in the American Western frontier, which is more in line with media stereotypes than, for instance, the Iroquois in New York or Seminoles in Florida.

Comanche were the stereotype. The memory of the Comanche empire would have been fresh in the minds of Americans (and Mexicans) in the classical hollywood era. Comanche held territory from Kansas all the way to Mexico. They literally destroyed Northern Mexico. Europeans have no idea how intense life was for anybody that had to live in and around that tribe. And this was in the 1870's (not the 1400's).

Perhaps you should see a seminal film before making sweeping judgements of American film portrayal of Native Americas? I'm not sure what your reference to the Iroquois and Seminoles has to do with it. Certain tribes are in American awareness more than others. That happened pre-TV and modern-day media. I don't think you get to decide who Kevin Costner writes films about, but you could write your own.