It is really inappropriate. Read some of the blogs on this. http://nock-knock.tumblr.com/post/34539114736/the-annual-halloween-psa-to-moniyaw-who-didnt-learn
It is about as historically offensive as dressing up as Hitler and a Jewish prisoner. It is perpetuating a stereotype that deeply offends people. Read this blog http://nativeappropriations.blogspot.co.nz/2012/10/so-you-wanna-be-indian-for-halloween.html#more
It shouldn't matter if you don't 'get it' it is not your culture to represent. If you are asked to not do do it, don't because it genuinely reduces someones values about their own identity or reminds them of past and present ignore injustices. It may mean little to you or you may get angry because you should be allowed to do as you please, but think critically about why people say it is offensive. It is not empty rhetoric.
or Tatum, B. (2000). The complexity of identity Who am I In M. Adams, W. Blumenfeld, R. Castañeda, H. Hackman, M. Peters X. Zúsmurf (Eds.), Readings for diversity and social justice An anthology on racism, anti-semitism, sexism, heterosexuality