Incidentally tips are like this for many regular non-gig-economy food service workers too - many areas allow an employer to pay less than the minimum wage as long as the total pay (including tips) meets the minimum. So a person who gets $0 in tips gets (say) 7.25/hr from their boss, but if they get 5/hr in tips they might just get 2.25/hr from the boss and not come out ahead at all - your tips are just a fee being handed indirectly to the boss. Of course, if they get more than 7.25/hr in tips they're coming out ahead, and there's still a minimum pay threshold so they're always getting something from the boss.
According to a quick search federal standards in the US requires at least $2.13 per hour be paid to employees who receive at least $30/mo in tips, with the total needing to hit 7.25. It's probably higher in some areas. This is part of why wage theft is such a pervasive problem in the US because it's very easy to get away with it in scenarios where you're supposed to be compensating for variable tip income, etc - sometimes the boss just doesn't pay what they're supposed to, oops!
The other problem with places that allow this is that if you're not bringing in enough tips, you effectively get one hour's worth of min wage followed by termination in practice.
