Yes, it's true that wait staff in U.S. restaurants are paid less than minimum wage.

And yes, it's true that tipping is generally voluntary in most U.S. restaurants and that the standard tends to be 15%.

On the other hand, if a restaurant indicates in its menu that the check for parties of more than x will be increased by a tip or "service charge" of x%, it could certainly be argued that the prior disclosure of the tip or service charge obligates the customers to pay the additional charge.