Why am I supposed to tip a waiter for bringing me food when I'm not expected to tip the flight attendant on an airplane for doing the same thing?

Why am I supposed to tip a waiter for bringing me food when I'm not expected to tip the flight attendant on an airplane for doing the same thing?

Research published in the Journal of Applied Social Psychology found that a diner is more likely to tip a waiter if the bill comes with a piece of candy, for example.