It seems to be the norm in Greece as long as you dont eat your own food/drink and it is polite to buy something from their own bar.
I remember our first trip to Greece years ago, we were on a bike ride, saw a nice pool thought, well we can use it, went and used their nice loos, had a nice swim, dried off and got back on our bikes. Only after did we realise why we got such black looks! They must have thought we had a right cheek.... we didnt buy a drink or anything we were quite ignorant of the custom to do that.
LOL