Call it political correctness if you will, but using humor to demean a whole class of people leads to an erosion of respect for them. Every time a lawyer tells one of those jokes they aid in that erosion. It doesn't matter if you are telling a blonde joke or a Polish joke, you are sending a message that the people you are joking about people who deserve to be demeaned in some way. Jokes lead to hate. It's as simple as that.