That’s a good question, and the easy answer is ‘they should.’ As the commenter above you mentioned, they use it as a tactic to advertise the same (competitive to other local restaurants) price people are used to. A more transparent way of doing business would be raising the price of the menu items to compensate staff fairly. The restaurant owners/management fear that if they do this it would drive away customers who believe the food is overpriced and look to their competitors. It’s easy to say, ‘just pay the staff a fair wage,’ but not quite as easy in practice. Most restaurants are small businesses just barely scraping by. The OP is right to be annoyed, but as always, context and a basic understanding of a situation’s underlying principles make the easy answer difficult to implement.