Gee... wearing an anti-Israel shirt in the middle of an AIPAC convention could cost you a tipBusboys and Poets. The waitress in question wore a t-shirt that said "occupation isn't pretty." The shirt was sold by the radicals at Code Pink. As one of the commenters at that link said, "Not exactly a "pro-peace" shirt. I'd probably can an employee who wore something like that in front of customers."
I wouldn't leave a tip either. (The diners in question apparently left a smaller tip than they would have otherwise).
UPDATE 12:29 PM
Here are front and back shots of the shirt (Hat Tip: My Right Word).