More often than not, folk audiences are a fairly sensitive bunch to when to sing and when not to. If someone is doing a free-floating, atmospheric, taking-liberties-with-the-meter rendition of a song that lends itself to that sort of thing (eg She moves through the fair) then it's unlikely anyone's going to try to join in.

Whereas with catchy chorus songs, you'd have to be a performer with a Van Morrisonean ego to think it was spoiling anything. (And it would also rather beg the question why any such performer would be choosing to play such rousing, inviting music in the first place...)