I would feel ethically obligated to give the waiter a tip.

From the legal standpoint, I imagine it would play out like this:
911: What is your emergency?
call: Help!! I've been robbed.
911: Are the perps on the premises?
call: No. They walked away.
911: Were they armed?
call: No.
911: What was the robbery?
call: They left without paying after I refused to make an impression of their credit card.
911: Did they give any explanation?
call: Yeah, they said "Sorry, our ATM's not working".
911: Click! [dial tone]