The companies are required to pay their workers at least minimum wages if wage + tips is less than minimum wage. As far as I can tell, tipping is justified by guilt-tripping everyone by saying "the food industry doesn't pay its workers enough so it's your responsibility to make up for the difference" instead of saying "the food industry needs to pay its workers a real wage and give customers the real price". If tip is not optional, then make it not optional, but don't guilt-trip people into feeling required to do something optional.