We cannot know with absolute certainty why people come to believe certain things or to have certain views. We can only speculate. I would argue that Americans see Native Americans in these contradictory ways because our views of Native Americans are based more on what we need to think than on what Native Americans were/are really like.
To some degree, it is important for many of us to believe that the Native Americans were savages. If we believe that they were savages, then our conquest of them becomes much more justifiable, at least in our minds. We can tell ourselves that these people were really bad people who deserved to be subjugated. We deserve their land because we are a superior people to them not only technologically but also morally. Therefore, we hold this view of them because we need to feel that we were right in taking their land.
However, in today’s world, some Americans have come to need different things. Now, these Americans need to portray our ancestors as somewhat brutal and ourselves as much better people than our ancestors were. Again, we use the Native Americans. We say that they were noble savages and our ancestors did not understand them. That shows that we are more tolerant. We say that they cared deeply for the land and were more in touch with nature. That gives us a way of promoting our current attitudes towards the environment.
In other words, the contradictory views that we hold of Native Americans have come about because our attitudes towards ourselves, towards non-whites, and toward the environment have changed over the years. This has changed how we need to see Native Americans in order to fit them into the story of our history.