While I'm sure most restaurants stay legal, there's a non-zero number of restaurants with owners that don't make up the missing pay and let workers end up making less than minimum wage, but the workers don't complain because they're truly desperate for work, or their local government won't do anything about it.
Now I'm remembering the Amy's Baking Company episode of Gordon Ramsay's Kitchen Nightmares where one of the owners, Samy, admitted that he takes all the tips his wait staff make, despite being illegal (Source: https://www.dol.gov/whd/regs/compliance/whdfs15.htm)
I feel a good first start would be to get rid of the tipped-job-exception from the Federal minimum wage law and set it a fixed minimum regardless of job classification.
In the United States, pay for tipped employees is regulated by the Fair Labor Standards Act (FLSA), which among other things requires that they always be paid a minimum wage.
https://www.dol.gov/whd/regs/compliance/whdfs15.htm
If anyone is claiming that a particular restaurant violates the law by paying lower than minimum wage to their servers, then that's the problem right there, not anything to do with tipping or tipping culture. If anyone is not being paid minimum wages they are owed, the US Department of Labor will be happy to intervene:
And yes, it's illegal to fire someone over reporting, but it happens all the time. And it takes time for a lawsuit to go through, so you've gotta find work somewhere else in the meantime. And now you have to also take time to sue your old company. When you're barely afloat, it's not worth rocking the boat because you might drown.
That's reality for a lot of people. You can't just say, "this is the law so it's not happening."
This is thoroughly false. It is so customary in the US that it has been baked into the minimum wage laws: in general, the federal minimum wage is $7.25, but for waitstaff and some other "tipped" jobs, the minimum wage is less than a third of that, at $2.13. The whole system is completely appalling, but that doesn't change the fact that in a lot of places the waitstaff is effectively subcontracted out and paid for by the patrons rather than by the nominal employer.