Wait, what? Thanks for going all "think of the children" on me man, where if I'm not with you I'm for murdering the natives. You are (apparently) representative of the moral absolutism problem I am talking about.
I'm not defending the extermination of native American natives.
The problem with this article is that it cast native American society as a utopian paradise, and the white European invaders as evil, ruthless, and completely without moral compunction. This sort of historical revisionist "blackwashing" doesn't help anyone - because it has not fairly represented either side. The natives were not utopian societies, nor were they savages deserving of murder. The Europeans were not ruthless monsters, nor were they benevolent conquerors. To represent this situation as black and white is both ignorant and unproductive.
By "blackwashing" I mean skewing the facts and injecting calls to emotion where it doesn't belong, to try and make something seem worse - where whitewashing is the opposite (skewing the facts and injecting emotion to make it seems better)
[edit] Also, to address specifically why the moral absolutism is a problem: by casting the Europeans as ruthlessly evil, you have removed any deep introspection into why the murder of the natives occurred. You've transformed a complex human situation into a fairy tale - where the bad guys need no explanation. You've also effectively discounted the possibility that bad things can happen for relatively benign (or at least, not overtly evil) goals.
Not to mention dehumanization is inherently dangerous - even if the person you're dehumanizing is the "bad guy".