What's the Joke?

I finally got a chance to read Gwen Ifill's justly praised op-ed on the Don Imus matter:

For all their grit, hard work and courage, the Rutgers girls got branded “nappy-headed ho’s” — a shockingly concise sexual and racial insult, tossed out in a volley of male camaraderie by a group of amused, middle-aged white men. The “joke” — as delivered and later recanted — by the radio and television personality Don Imus failed one big test: it was not funny.

This is a good point. The scare-quotes are essential. What was the joke here? One can imagine such a thing as a racist joke. There's an anti-semitic joke I like that goes "what's the difference between a Jew and a canoe?" Answer: "A canoe tips." That's a joke, albeit an anti-Jewish one. "Nappy-headed hos" isn't, as far as I can tell, an actual joke any more than "George Soros is a rich, greedy kike" is a joke (get it?). It's just a racist insult.