Pretty sure they’re only guaranteed minimum wage if they don’t receive tips. So yeah if you’re cool with your waiter making $7 per hour instead of $3, don’t tip.
Now you’re raising two different issues. The first is a truly abysmal minimum wage. The second is a lack of effort on the part of both the staff and the employer to negotiate an acceptable minimum wage, whether it includes tips or not, enabled by the reliance on tipping to provide an acceptable wage.
I’m not aware of how things are on the ground in the US; it’s more of a general perspective not tied to any specific country (which is my separate “meh” about strongly America-centric Lemmy, but it’s very tangential)
But seeing people actually hit the absolute mandated minimum is indeed depressing. I might expect that waiter’s income with tips is normally greater than the federal minimum, so maybe there’s a chance not all waiters will agree to work for that, which should balance it a bit?