I don’t really see the comparison between Don Imus talking about “nappy headed hos” and hip-hop artists rapping about “bitches” and “hos”. I don’t see US Senators and major political journalists doing guest appearances on rap albums and praising misogynistic rappers as praiseworthy sources of information on weighty topics. Indeed, quite the reverse. Politicians generally enjoy hanging out with celebrities (enjoy it a bit too much for their own good) with shy away from rappers for precisely this reason.
There are also speaker/author distinction issues in play, but that maybe gets things too complicated. As for Imus, I wouldn’t cry if he got sacked, but I don’t think that’s absolutely vital either. It’s his status as a media and political power-broker and member of the “respectable” establishment that’s totally bizarre here. There’s a lot of offensive material slushing about in the media environment, but little of it is so explicitly ratified by these kind of people.