I've seen this happen soooo many times. Just why do people think it's ok? To clarify, it does not happen with good friends usually. It's usually when there's a 'party' of some kind or meal made up of either work colleagues or mums from some kind of school or hobby group.
Everyone goes out, some people know each other better than others and earlier on in the evening a sizeable number of people slink off leaving others to foot the bill. Quite often it's those who can easily afford to pay!
Has this ever happened to you? Have you ever done it? Because it seems to happen quite a lot ime!