There's a certain insincerity in behaving differently because of who is the organiser/promotor/etc

I think the idea is that if the organiser has a word, he or she is telling you to shape up for the benefit of the singaround as a whole, whereas if another punter complains they're effectively saying "you should sing better for my benefit".

Interesting point about getting up mid-song - yes, that would be seen as mightily rude at all the singarounds I've been to, and some of the folk clubs. People do make their move when somebody's starting to sing, but obviously that can only express a reaction to the singer (based on past performances), not to this particular performance. For that we'd be dependant on the yawn, the shuffle, the noisily-deposited glass, the muttered comment to the neighbour, the half-hearted chorus contribution and (worst of all) the polite ripple at the end of it. (I've had 'em all, needless to say.)