If you'd check the Michelin guide, you'll see that the Nobu restaurants are some of the best rated in the world. Getting a job as a waiter there is extremely difficult, and the managers wouldn't put up with anyone of the staff giving anything less than excellent service. Seriously, that sort of tip is an insult, though given the way some celebrities seem to think of themselves, Piven probably thought that he was "giving of himself."
Mark
Any restaurant that allows a waiter to throw things at a customer who insults him is probably not the kind of restaurant I'd ever visit anyway. Must be a five-star thing I guess.