I suspect that those who view it as a slur are either: (1) ignorant of history or (2) upset by the descriptive adjectives that later came to be placed in front of the "redskins" word. Given the historical record, the word itself is not a slur.http://itre.cis.upenn.edu/~myl/languagelog/archives/002961.html
According to this article the term "Redskin" is a translation from a word the Indians came up with to distinguish themselves from the white man. I'm not sure of the credibility of this particular article.
However, Colin Cowherd had a historian on his show a few months ago to discuss the origin of the term. The term (according to the historian) was created by the Indians themselves and not by the white man. So if any of the Indians are "offended" by the term then they need to blame their ancestors.