They don't know how to tip properly
They spent their summers in Nantucket at their family's estate or they were shipped off to Camp Kieve or equestrian camp, or a pre-college program at Oxford or Cambridge if they were tacking on accomplishments for college. But their parents never encouraged them to barback or bus tables to learn some work ethic. In fact, their parents would never encourage them to ever take on a job that pays by the hour ever. So when it comes time to pay the bill, they have no idea that you're traditionally supposed to pay 20%. Or even worse, the waitstaff makes a mistake and they refuse to tip because they're oblivious to the fact that waiting tables is actually one of the most stressful jobs in the country (no seriously, studies prove it).
[Photo via @tasteofthevineyard]