If a song is well known, it may be perceived as the common property of some or all of the people present and therefore legitimate to join in with.

I prefer to sing songs that otherwise won't be heard because either they are not sung at all (as far as I know) by anyone else in that particular place or they are only sung in completely different versions. Despite that, I sometimes notice that someone else knows the song and is joining in. I find that disconcerting, but not because it affects my singing: only because I realise that it is better known than I thought it was.

Richard