Sometime you think the service provided by a waiter or waitress doesn't deserve gratuities, but you still offer them to him anyways.

However, the restaurant probably won't see you again.

Are you fair to the waiter or waitress? Are you fair to the cook who might have cooked you an excellent meal?

In that sense, the waiter or waitress is sort of a salesperson, and he shouldn't have an entirely fixed pay for his work by the restaurant.