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)
I think the caveat has been made on this thread before, that people could just be having a bad day, and so it's then nice to read about counter stories. There are however certain slebs that seem to recur quite a lot in unpleasantness.