When is it considered socially acceptable to joke to a stranger that people like you should all be dead?

Answer: When you find out someone is a lawyer.

I learned this little point about the expression of hatred when Christmas shopping today. The salesman saw that I had an American Bar Association credit card and proceeded to tell the ancient joke about what you call a large number of lawyers in a crashed bus at the bottom of a body of water, with the answer being "A good start." He was Madison enough to make the body of water Lake Mendota.

Now, you could substitute any group for lawyers in that joke, and I'm sure the joke has had many versions over the years, used to express hostility to all sorts of groups. But the only version I've ever heard is aimed at lawyers, because apparently it's just perfectly fine to say anything nasty you want about lawyers. But here I am, buying Christmas presents at the man's store. How about a little "Merry Christmas"? Or even "Happy Holidays"? What the hell, I'd settle for "Seasons Greetings"?