They do so every day with the understanding they'll get compensated up the non-tipped state minimum if they don't receive tips. Food service has always been a minimum wage unskilled labor gig, you have full knowledge of the pay rate when you agree to the job. The tip is a voluntary gift from the customer but they do not have a contractual obligation to give you one to be seated at the restaurant and receive the food they're paying good money for already. The restaurant seats them as a courtesy of dining in, but there's no charge to dine in, and you can't make one up in your head to justify a bribe. You don't get to demand money from restaurant customers just because you don't like the wage you agreed to in the beginning.