Note (if you don't know why someone would be against it: http://nativeappropriations.com/2011/10/open-letter-to-the-pocahotties-and-indian-warriors-this-halloween.
Edited to add the letter I wrote:
Why I won’t be dressing as a Native American on Wednesday:
I’ve spoken to some of you already, but this is an issue I feel is important enough that I’m choosing to write a letter to you all. Dressing up as a race that faces daily oppression and hatred is derogatory and not okay. I like to believe that the majority of you would never consider dressing as a Jewish person or a Black person as a costume. Dressing as a Native American is the same. You may not have bad intentions when you wear a headdress or ‘native prints’, but it doesn’t matter. The act of encouraging the stereotype does damage in itself. We are poorly informed about racial issues in the UK, let alone ones in other countries, however ignorance does not excuse or lessen the hurt that it can cause.
To some, ‘Cowboys and Indians’ is just a genre or a game. But it glorifies the eradication of history and culture that far predates ours. We don’t understand what it means to have our culture turned into a joke, a costume, to have things sacred to us ruined and taken away. The majority of us are privileged, if not from good upbringings, simply from being white and never having to be judged by the colour of our skin.
If you don’t want to listen to me, that’s fine. Here are some wonderfully written pieces by Native American authors. If you choose to still dress up on Wednesday, understand what it means to take an entire race of people and turn them into a costume.