Having worked several waitstaff jobs, I know for a fact that you can legally be paid below minimum wage. And although you rarely, if ever, make below that, they are not going to increase your pay to 3 bucks from the usual 2.50 on the nights when you don't make it. Trust me, they don't and they don't have to. There is a legal loophole here because some nights they will make below minimum wage. As long as it averages out over a pay period, then it is legal.

Ask a waiter sometime. They'll give you the scoop. If you, or someone else you know has gotten more salary on the nights when their tips didn't make it, it is the exception, not the rule.