It should remain voluntary.
No-one is going to work for $2.13 an hour, and nobody does.
Well, being an independent lot, we Americans seem to think the whole tipping thing is up to us. It suits ‘our’ personality.
No one does work for $2.13 an hour .. that’s the point, BUT $2.13/hour is what restaurants/bars pay their waiters/waitresses. Customers’ tips make up the difference (which can be significant).