It's easy to get away with it, is one reason. Indians can, and do, look like anyone. And anyone can look like an Indian. After 500 years of intermarriage, Native American racial identities (as opposed to cultural identities) comprise a wide range. Among my three siblings, one of us looks like Opie Taylor, one like Tonto, and one is a dead ringer for the Karate Kid. (I'm Opie. Opie is my spirit guide.). . . .
Tragedy is a shortcut that sells, and the particular tragedy of being an Indian has an amazing ability to make readers lose their capacities to discern good writing from bad, interesting ideas from vapid ones.
And, in Ward Churchill's case (though he isn't mentioned), historical truth from polemical fiction. Whole thing good.
Update: Opie and Ward Churchill: separated at birth?