So the waiter has plausible deniability if he is faster than you. Anybody in the room could have taken a picture of the receipt while falling out of the printer, spied on the electromagnetic waves the printer produced etc.
(Yes I'm one of those skeptics who think it is a bad idea to print private keys in the first place. Bitcoin are digital money. Printing them is a nice gadget but of no practical relevance in any future I can imagine.)
In a BTC-welcoming restaurant scenario, you are far more likely to receive your change as multiple BTC bills pre-printed by the restaurant in a small denomination conducive to leaving tips, with scratchoffs protecting the private key. Your restaurant receipt will list the firstbits of the change bills that were activated for you as proof that the bills were funded by the waiter before being brought to you.