I'm pretty much in favour of joining in (unless an unaccompanied singer has specifcally requested no instruments), but it is very much up to the joiner to make sure he or she can do the necessary, and keep with the original carrier of the song/tune. Having some arseholes is the price of participative music, but there again some "singers" who account it a right to bore/embarrass the arse of us almost inaudibly in varying keys and tempi do need to be told firmly to get over themselves.

Perhaps these good squeezers genuinely thought the singer had in error started in a key that was going to make matters hard for HIM (the singer) and were trying to help. Perhaps they weren't.