It was in many ways "A Man Called Horse"
I loved the move but the plot has been done to death.
My wife pointed out it was that it was a typical romance novel as well as a typical 70s western.
The Romance novel was "man pretends to be something else, man falls in love with woman why pretending to be something that he is not, woman finds out and gets mad, man becomes what he pretended to be and they live happily ever after."
Also you the Western theme of white man falls in love with chief's daughter.
So yea the plot is your typical white man's guilt vs the noble savage.
As to it being a retelling of the story of Native Americans? Not in any but the only most superficial kindergarten way.
The variability within the tribes means that there isn't really one Native American story at all. The myth of the noble savage is just that a myth. Some tribes where every bit as nasty as the worst of the white men of the time. Many of the tribes had no problem with rape and slavery as methods of control and domination and some used theft as way of life. They had not trouble take what they wanted from other tribes by force. Their culture was totally at odds with what we consider right. Other tribes where much closer to the ideal you see in the movies.
Back to why do we like this movie? It is a good movie and a good work of fiction so why not like it. If you want deep meaning from a movie or if you find deep meaning from a movie you really need to stop going to movies and pick up a good ten or twenty good books instead. Movies are a few hour emotional quick trip and not the path to enlightenment. Even the best of them like Schindler's list are not that deep when you think about. How deep is the concept that actively saving innocent people's lives even at the risk of your own is a good thing?