You pose a great question. No joke is any good unless it is somewhat offensive. The most basic slapstick involves people falling down, getting hit over the head, etc., and from there it goes to things like the jokes to which you refer.

IMHO NOTHING is out of bounds, and every topic, race, creed, religion, whatever is fair game for comedy. This does not mean it is not offensive, nor does it mean everyone has to be forced to listen to things they don't like. But like so many things, with comedy there is no way anyone can start drawing lines.