Dick - I have no intention of following you into one of your rather gloomy and unpleasant blind alleys - Mike has just said it all
"It is surely a matter of courtesy to take one's lead from the performer"
If a performer is happy that an audience joins in on what he or she does - fine, that is their prerogative.
Nobody should ever be put in the position of having to ask an audience not to join in; that is an unacceptable imposition and where a club carries it out as common practice, it would set up an immediate barrier between listener/would-be-joiner-inner and performer.
On the few occasions I have found myself sitting next to one of these strange people in an audience I have moved away rather than ask them to shut up (my first instinct)
People here have argued for it happening at open sessions, they have suggested that it is arrogant to expect to be allowed to sing solo.
Personally, I would avoid such clubs like the plague - they should be required to put up health warnings - it is, at the very least, bad manners - at worst artistic vandalism
Jim Carroll .