If we see the Native Americans as murdering rapists, how likely are we to give them restitution for the massacres we've inflicted upon them, or begin to provide them social support? And if you don't currently see them as such, how will fiction that impresses on the greater public of its veracity that it isn't so, and that the general impression of their culture is completely fabricated?
Fiction definitely shapes our society and our understanding, and if fiction as popular as this one claims to be realistic there is a responsibility to either confirm or counter it. Yes, storytelling is more important than accuracy, but when you claim the latter you had better prove it.