Why does everyone get so bent out of shape about dressing up like (american) Indians?

Just read this: http://www.cnn.com/2012/11/30/living/native-american-fashion-appropriation/index.html?hpt=hp_c1

OK, I realize that the native americans weren't always treated fairly. However, why do people get upset and petition about the Cleveland Indians, Chief Illiniwek at U of I, the Redskins, a beautiful Victoria's Secret model dressing up, etc? I think that the idea of Hooters sponsoring a cowboys and indians night sounds like a lot of fun!

There are plenty of other ethnic groups that are portrayed in our culture and I don't see the same backlash. What's the other side to the argument, and is it warranted?