Ebert grew up in Urbana, Illinois, where the University of Illinois is located (it actually straddles Urbana and Champaign). In his essay, he talks about African American children in his school and in his childhood.
He understands racism where African Americans are concerned, but he seems to be conflicted over stereotyping of American Indians. In a 2009 essay at his blog, he said that "Chief Illiniwek" is "the world's greatest sports symbol". Following his essay is a video of the mascot's "last dance".
In the comments section (he got LOT of comments), he says:
The Chief. *Sigh* I understand intellectually why Chief Illiniwek was retired. I agree with the decision ideologically. But my heart cries out, as in my memory he stands proudly on the 50 yard yard line and the Marching Illini conclude the school Song, Illinois! Illinois! Illinois! He was so much more dignified than a buckeye, a wolverine, a badger, a boilermaker, a spartan. He was greatness. I'm glad I was there.
His emotions and his intellect are at odds. He can't condemn "Chief Illiniwek". Based on my understanding of all he says in the post itself about African Americans and race, I don't think he'd say that his heart cries out for the old black and white minstrel shows. I wish he had that same insight for American Indians and our objections to stereotypical depictions like "Chief Illiniwek".
Update: I'm adding another comment from Ebert that pre-dates the others above. The comment below is from "Noble Spirit, More than Just a Mascot" dated 2001 the Chicago Sun Times.
"Chief Illiniwek, for nearly a century the symbol of the University of Illinois, was until recently seen as a positive image of American Indians. The Chief never was a 'mascot,' and indeed goes back so far that he pre-dates the use of "mascots" for most sports teams. ... In recent years, however, Illiniwek has been under attack from a small, self-righteous coalition that wants to wipe him from the university's history."