You can't compare Native American Tribe names/references to deragotory descriptions of other groups. No organization would ever name their team the "San Francisco Faggots", the "Houston Honkies", or the "New York N*****s." Not because these organizations are sensitive and considerate, but because groups aggregated under these terms have no virtue, honor, courage, etc. that is important as a metaphor or symbol of a sports team.
Conversely, a tribe name, though possibly offensive to some, does invoke some spirit of courage, power, etc. that sports teams want to be associated with. College and Pro sports teams named themselves after Native American tribes to tie themselves to the warrior past of these tribes.
Its kind of pathetic, but after we wiped out the plains indians, and white Americans were safe, we started turning them into heros. Nonetheless, referencing Native American culture as a mascot/nickname is meant as a tribute, regardless of what its denigrated into today. Like Craig said, is the name Seminole in itself offensive? Maybe, maybe not depending on your views. But everyone would agree racial slurs for blacks and homosexulas are offensive--and also--meaningless with respect to symbols sports teams want to be associated with. So its comparing apples and oranges.
Here's a question: Lets say the Oakland Radiers were reanmed the Oakland Zulus and the San Diego Chargers were renamed the San Diego Highlanders. The Zulus had a weird looking black man with a spear and reed shield dancing around his sideline and the Highlanders had a weird looking half naked painted Scotsman with a sword dancing around his sideline. There were never any Higlanders or Zulus in America. So would any of this be offensive?? (Being a decendant of one of these groups doesn't count because they're in a different geographic region).