"It is surely a matter of courtesy to take one's lead from the performer",
exactly the point that i made, the performer encourages or gives a lead, or if he is in a situation where something he doesnt like occurs, he has every right tactfully to ask the person to desist, whether it be a club or singaround or irish song session, I had to do this with a tourist who was not english who was playing random notes on a harmonica to a set of tunes, at an irish pub session, i explained nicelt fuirst time that it was a good idea to know the tune but if not to play quietly,[he took no notice]so i had to ask him again, this guy was just playing anything regardless of the tune or key, but that is a bit different from people repeating the last verse of a song.
I am in effect partially agreeing with you , with the proviso ,it can vary on ciurcumstance.