Good point - most states require the pay to be the greater of 1) normal minimum wage OR 2) wait-specific min wage + tips/hour.
So, a waiter should always earn at least minimum wage, but can earn more if tips are good.
However, in practice, there are problems with this system. First, waiters who don't earn lots of tips usually get the bad shifts, then fired. Second, lots of room for wage theft.
And there is a whole bunch of wage theft - you can't forget that wage theft accounts for the most damaging crime by pure value in the US alone. Individual occurrences aren't generally for tons of money, but it happens everywhere.
So, a waiter should always earn at least minimum wage, but can earn more if tips are good.
However, in practice, there are problems with this system. First, waiters who don't earn lots of tips usually get the bad shifts, then fired. Second, lots of room for wage theft.