Zimmerman
"respect the singer."
You have that right.
Have just been listening to an interview of the great Sean Nós singer Joe Heaney where he is asked precisely the same question you asked - his reply "One singer, one song".
Our folk songs, by and large, are constructed as solo pieces which are made for individual interpretation. Joining in wherever and whenever an audience feels like is an infringement on the right of the singer to give that individual interpretation - how could it possibly be anything else?
Silence on the part of the audience is an indication of respect for the singer and an interest in the song being sung.
Some songs are obviously made for joining in - they come with choruses and refrains; but even these should be treated with sensitivity - don't turn gentle refrains into belted out choruses - let the singer make the running - at the moment of singing it's his or her song.
If audience members want their five minutes of fame, let them go and get their own songs - not being precious - it's good manners apart from anything else.
Jim Carroll