Probably related to the fact that the waitress can justify no tip with the thought "they might have forgotten to leave one". A short tip sends a message.

Which is a good point - as it gives illustration to mikeash's point in this same thread. This doesn’t make logical sense at all - but there are practices based on it.

And in this case both the "a token of our appreciation" and "short tip" practices could been "meant", and were as a matter of fact taken as "meant" by the sender and receiver respectively.

Oh hell, is it? Quite often I'm short on cash and just tip with the change I have - not tiny-denomination coins, I hasten to add, but 50ps/£1s. Bugger.