If you ignore the fact the various tribes stole from each other. Native Americans weren't a single, homogenous mass, and they certainly weren't Noble Savages ignorant of the ways of war.
If you steal a quarter from Bob, and Bob steals a marble from you, are you saying it isn't theft (morally) for me to come along and take your quarter and Bob's marble and all your other toys while I'm at it?
Is it stealing to exchange a grain of sand for your quarter? The Dutch bought Manhattan for around $1000 adjusted.