I can understand Mr. Carroll's concern that folk session is potentially muddied by introduction of non folk song.

But unfortunately this is time we are in. Singers age and die away, taking the traditional songs with them. New people come along that have not been indoctrinated as others have been, changing what may have once been a totally traditional song session into something else. This is not necessarily bad.

If rendering a pop song in a session draws a youngster into the room and that young one stays and discovers the next traditional song a wonderful revelation and goes on to find out more, invite friends, etc., then that "muddying" of the session has served a positive purpose. The above is not a judgement of the quality any song brought into a session, only a statement of how things are and can be.

Getting back to the original question when should singers not sing. It will always be a matter of consideration from both performer and audience. The performer can and should invite participation or indicate not. The audience (there is always the potential for beer soaked exceptions) should follow that lead.