Folk song has never been been about joining in - it is about people who have turned their emotions and experiences into musical verse and passed them on to others - poets, playwrights, authors.... painters whoever, have done the same for millenia and will continue to do so long after we have shuffled off.
I am totally at a loss to understand this moronic desire to join in with the verses of a song should have ever caught on - the massed choirs of the folk song revival.   
Try going to an opera and joining in - or muttering your way through Shakespearean soliloquies.... I've seen people being asked to leave a cinema because they insisted in echoing familiar lines in a film.
Joining in on songs as a matter of course without being asked is arrogant interference in a solo singers rights to interpret a song for themselves.
If it is a widespread trend in British folk clubs it is an extremely disturbing one - if it happens at singing weekends I attend here people talk about long after the culprits have departed from Shannon airport back to Heathrow (it seems yet another British eccentricity designed to damage folk song even further).
If I have been given the opportunity to sing I expect to be allowed to be given the consideration of people listening to what I an saying with my songs.
If I want them to join in I will ask them to do so - that's what choruses are for.
If you want to sing, wait till it's your turn.
You want to sing in groups - join a feckin' choir!
Jim Carroll