The joke is that Ward Churchill is not really an American Indian. He's a white man pretending to be one. That's what AIM (the American Indian Movement) says. He doesn't have any proof that he or any of his relatives are Native Americans (his in-laws are, but that's not the same thing).
If he really was a Native American and/or he really had the support of Native American communities in America, I think we would be more careful about offending people. But as it is, he's just a white jerk who wants to pretend he's got non-white blood because it's politically correct. He is trying to usurp the identity of genuine Native Americans, and he's harming Native Americans by associating them with his hateful remarks, which they don't agree with.
And, he's based his entire career on the concept that he's descended from a minority against whom America committed genocide. He is a complete fraud -- in the criminal sense as well as the social sense.