Imus has been a racist forever and a scumbag since before that. He also plugs the shit out of people's books, so they go on his show despite his obvious scumbaggery. That's how the world works. A few years ago he compared the animatronic gorilla effects in Instinct (good film) to "the starting lineup of the Knicks."
By the way, "free speech" means that the gubmint can't throw you in jail for something you've said. It doesn't refer to some social obligation to ignore stupid or offensive remarks, or to continue paying someone for making such remarks.