I accept that my attempt to justify such humour was wrong. That said, I stand behind my main point which is the reason it was being voted up was because people were amused by it.

It seems wrong to apply logic to people's perceptions of humour and offensiveness, since perceptions are rarely logical. But going with this, if the person telling this same joke were black, would that be OK? Or equivalently, are there classes of jokes that may only be told by black people?

(also I feel I should point out that jokes about race tend not to be equivalent in offensiveness to jokes about sex, so this whole premise is largely irrelevant)

I think those jokes are acceptable. I think racist jokes are funny, and that includes jokes that make fun of my own race. We laugh at absurdity, and one thing that's pretty absurd is racism.