What's the big deal with Chief Illiniwek?

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Why did this mascot of the University of Illinois have to be retired? Is there a shortage of true worries or concerns in this world that native american advocates felt the need to fight for the retirement of this figure? And how was it exactly that this mascot offended people of indian descent? I was under the impression this mascot represented the university. It wasn't a symbol of scalping people, or a symbol of hostile takeovers of indian reservations, was it? So, what's the real problem with him?