I'm always dubious about people claiming to have met rude celebrities when the celebs are off duty. I think I'd be pretty p**sed off if I was on downtime and someone came up to me expecting me to be "on".
Celebrities who are needlessly rude in a professional capacity, however, are inexcusable (just like every other person who is in a work environment)
If you're a celebrity you can't expect to never sign autographs for fans. You don't have to, but they are the people who keep you in work.