Good point, DMarks. But performers have been dressing up as Indians since the days of Wild West shows and vaudeville, if not before. They all could've claimed they were trying to be different, daring, and edgy.
Again, the situation parallels that of minstrel shows. I suspect audiences considered blackface performers terribly risqué in the early 20th century. "We don't allow blacks, but he looks so black...ooh, how titillating!"
Again, our culture has spurned performers who pretend to be black (or gay, or drunk, or fat, or mentally handicapped). As I've said before, Indians are about the last bastion of "acceptable" stereotyping. We've tolerated it so long that Kesha and her fellow wannabes think anything goes.
If you truly want to be daring, hipsters, dress up as Jesus or Muhammad and do something profane. Or dress up as Obama and pretend to be an ape. Dressing up as an Indian is about as daring as dressing up as a princess. It may be novel if you're a 2-year-old going trick-or-treating for the first time, but that's about it.
The Hipster Headdress Challenge
Here's a challenge for you: Which of these 10 images are a) daring transgressions of our cultural norms; b) bold statements about 21st-century tribalism; c) wholesome attempts to honor Indians; or d) ridiculous Halloween costumes worn by ignorant children? Explain why each image falls into one category and not another. (Answers without adequate explanations will be considered incorrect at my discretion.)
Good luck with your answers...you'll need it.
For more on the subject, see The "Honor" of a Plains Chief and Why Hipster Headdresses Aren't Okay.