Firms are responsible for their expenses, including the wages of workers. (Revenue - Expenses = Profit)

Tipping began, in the U.S., to acknowledge good service. It was intended to be a bonus. Today in the U.S., tipping is expected. Many restaurants pay relatively low wages for tipped positions because tips will (theoretically) bring workers' compensation to a competitive level. So...is a tip from a customer a subsidy for the restaurant? Should we tip or should we expect restaurants to pay for worker wages in full?