The simple reason is that they get no ratings in those hours, anyway, so turning on a couple of cameras while Imus does his radio show is cheap product. Also, there is no public indignation over Imus. He's such a smarmy kiss-ass to Senators of both parties that he covers the bases. Imus is a very "inside the beltway" phenom--he's the radio host DC puppet heads think is cool, for some reason.
LOL im to the right of Attilla the Hun and i listen to Don every once in a while and find him funny. Of course i can't stand his liberal leanings, but he is funny.
NBC should be ashamed of itself; but then again, they have no shame.