He's a shock jock. He was trying to shock. He was trying to be funny. Other people (witness this thread) say things so much worse than what Imus said. I don't get it.
Although, perhaps I do -- after Imus said what he said, he felt genuinely sorry. He apologized. He tried to make amends. Sharks can smell fear and from that moment of genuine contrition, Imus was a dead man.
Opie and Andy probably don't feel bad at all. So they'll be left alone.