But it's really hard to enforce, and if you were a waiter who consistently had to ask to be topped off to meet minimum wage, you might be putting yourself on thin ice with the manager.
As scarmig says, it's hard to enforce, but even if it is correctly enforced, it's on a per-paycheck basis, not on a strict hourly basis. So imagine a server who makes an average of $20/hour with tips (which seems reasonable) -- if suddenly two-thirds of customers become tip abstainers then the server's pay will drop down to right around the normal minimum hourly wage. The top off only gets you up to poverty wages (keep in mind many server jobs are part-time); any lack of tipping above that point is pure loss for the server.
Also, I happen to be in Madrid right now, which doesn't have a tipping culture, and let me tell you that I've had some pretty bad service here. Way worse, on average, than in the United States.
But it's really hard to enforce, and if you were a waiter who consistently had to ask to be topped off to meet minimum wage, you might be putting yourself on thin ice with the manager.