Actually, people that wait tables in most states make less than $3/hr base pay, which is about $20 a day. So yes, we already have people that make $20/day base pay + tips.
The article also states that they were allowed to accept donations, so the $20 number is probably quite low and I suspect they came away with quite a big more than that for each day's work. Minimum wage is about $60/day, depending on where you live.
If interpreted broadly (any source of financial support), zero salary means I am literally starving to death soon. I'd do a lot of things before letting myself die. But in practice, the opportunity cost of doing your 1 cent job is just too high, even for desperate people.
Basically you've already conceded that, we're just discussing the amount (to paraphrase something else). I feel it's already been adequately set by law.
The restaurants are also required to difference between minimum wage and hourly pay + tips if they don't reach minimum wage. Whether that happens or not is another matter.