There's a serious race debate going on with Avatar which I've kinda avoided. As to the question of why we keep seeing these "white guy goes native" flicks, I think the answer is pretty obvious. Historically, most of the people watching the movies have been white (I bet that's changing) and, most importantly, the people making the movies are themselves white. When the natives get cameras--or when we're all natives--then the natives will make flicks about saving themselves. Until then, we're probably doomed to be passive scenery.
More interesting to me is the constant comparisons with Dances With Wolves. I need to go back and see this flick, but I remember liking it a lot and feeling like Kevin Costner was actually pretty respectful of the "natives" and their traditions. But it's been a long time and I could be wrong. One scene that sticks out for me happens toward the end. Kevin Costner is captured and on his way back to be tried. But the natives actually save him, instead of the other way around.
I haven't seen Avatar, so maybe the comparison is apt. But I never thought of Dances With Wolves as "white guy saves natives." I thought it was clear, from the start, that the native were not going to be saved.