The question isn't so much whether the terms and images are offensive to anyone, as much as they can be viewed as a cultural appropriation. Imagine if your group of people was nearly exterminated over a period of 500 years, and then those who subjugated you started using names, images and tropes that were yours, for their own wholly inappropriate purposes, even if it's done as respectfully as possible.
I'm not suggesting I'm 100% onboard with that attitude, but it does make one stop and think. Truly offensive names and images (like the Redskins, or the arguably racist stereotype that's the Cleveland Indian) are only the most egregious examples. Being from Illinois originally, I was shocked that the U of I was pressured to eliminate the Chief Illiniwek character from university sporting events, when he was presented fairly authentically and with a healthy dose of respect (notwithstanding the gymnastic tumbling, which I'm sure was never part of the real chiefs' repertoire). But the more I thought about it, the more I realized it's about more than just demeaning caricatures.