At bars on the other hand the 'corruption' is much clearer. Tip badly for a round of drinks and you'll get slow/bad service for the rest of the evening. Tip well and you'll get priority service and perhaps even free booze. Especially with spirits and mixed drinks the corruption is obvious. Tip well and the bartender will pour you a double while charging you for a single, essentially 'stealing' from his employer and selling you the stolen goods at a discount under the table.
It's not totally implausible that the owner is fully aware of the situation and accepts the status quo as a local maximum of profitability, why mess with standing social norms -- much how most restauraunteurs in the states wouldn't dream of paying a living wage for their wait staff because they know customers are going to tip sufficiently regardless of wages and that enough potential employees are fine with the situation (or prefer it) that they won't have staffing issues.
TLDR: It’s remarkable what tipping and basic human decency can do to the quality of service you receive at a bar.
Of course we now need an experiment to disentangle those two variables! One team tips very well, but treats the staff badly , one team treats the staff with plenty of human decency, but doesn't tip. Then we measure the quality of service received by the two groups. I wonder if I can apply for a research grant?
The point is, fundamentally the current way tips are collected in the US is like corruption. And like corruption, it should be made illegal.
For those who think people can’t survive without tips, first it is a problem that they are mistreated by their employer, second make it legally binding to mandatory add 20% or whatever to the meal and pass that to the directly (again legally binding).
If you think tips encourage good quality of service, other places without a culture of giving tips have our ways of providing good quality of service. You don’t need tip for that.
I once heard tips people gave, that when checking into hotel rooms, you should hold a 20 dollar notes as hint of tip and then ask them for upgrade. If they do, then you give them the tip. This is bribery, one of the many examples how tip culture can be used in a way indistinguishable to corruption.
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.
Today, tips are paid because wait staff are paid below minimum wage. This is legal - as tipping is baked into their compensation. It's stupid and anachronistic, but it's not the fault of the staff (though GOOD waiters will often express support for the system because they fear going to fixed wage w/ zero tipping will reduce their income).
I'd much prefer a living wage with optional tips for extraordinary service (which seems to be the norm in much of western Europe). Here in the US, 20% gratuity is pretty much expected. And FWIW, I'll pay 25-30% for good service, and to get less than 20%, a waiter would have to be downright awful.