It's the norm in many states, so I don't think so. I grew up in IL where waiters make something like $3 / hour. If that + tips is less than minimum wage then the company has to make up the difference. Not quite the same as they don't dock your pay if you make good tip money, but same principle.
It works out to the same end result (with the exception that waiters usually have some absolute minimum wage they can earn, so if a waiter makes 20$/hr in tips the restaurant still need to pay a bit on top of that.
One super annoying thing about this system and people choosing not to tip is a situation like this, assume a waiter is serving two customers in an hour, the local minimum wage is $9/hr and the sub-minimum wage is $3/hr, the first customer tips $4 dollars, yielding a potential wage excess of $2 (assuming a reliable rate) when the second customer tips $0 then the waiter ends up making no money beyond minimum wage. So if you're a tipper another customer that doesn't tip can cancel out your tip.
I'm not certain how these are aggregated from an accounting perspective, but I wouldn't be surprised if the window was either a full day or a pay period (in the latter case, every two weeks someone totals $3 * hrs worked, adds on total tips and verifies if that number is above $9 * hrs worked (do nothing) or is below (make up the difference out of the employer's pocket.
