^ I ain't never been no good with logic jokes, but I don't half not get that one, oh no.
EDIT: I think now I got it. It's about the joke's title. Ok ok ok... But does sarcasm count? It's only a negative with the correct intonation, isn't it? And the linguist in the joke is of course only right about the double negatives if he ignores loads of non-standard Englishes...