The difference is that a tip is "you're already getting paid to do your job, why would I pay you to do your job?". Whereas this is "you gave me this for free, so maybe if I make a bunch of money I could show appreciation for that gift" or pay-it-forward so the next guy can also get a free start, etc.

And yes I know they're are busted places on this Earth where basically servers don't already get paid to do their job and are thus reliant on tips as income, but that's a different problem...