1   Why do people feel they need to joint in with a singers songs unasked?

Because they enjoy doing it and that is useually what they are present for, entertainment

2   Do they feel they have the right to do so automatically, if so, why?

Obviously they do feel the right; why? I think it's culture since so many people grow up singing to radios, tv, record players, and now ipods, etc, etc.


3   Does this right aapply to all songs - if not, which songs are exempt

one they like, sure.

4   Don't they consider it the height of bad manners to put the onus on the singer to ask them not to join in?

nope.

5   Don't they think that people who 'pop their cheeks' on certain songs should have their fingers removed surgically - without anesthetic

have no idea what you mean by this.

As far as your supplementary question - those people who do so will do so usually no matter who is performing. I wouldn't - but I can't play an instrument.