I find the use of the term 'police' in this context almost designed to provoke more heat than light. I am not in favour of anyone dictating 'in the large' what should and should not be done to folk music; nor am I in favour of anyone assuming the past can be blithely ignored. (And that applies to a great many of my interests, not just folk music.) But in specific situations and circumstances decisions have to be made. That is simply life.

A Point Against Policing: They are hopefully aware of the impact Cecil Sharp had on recorded folk music. Again, most of them are aware of how William Kimber sparked at least part of his interest. William's instrument of choice was the concertina, which at the time was about as 'recent' as the electric guitar is to us. Folk music changes.

A Point for Policing: Club/session/singaround organisers have a duty to guide the way it develops so that they believe it is being strengthened. (Of course, they will sometimes be wrong!) Necessarily, that involves them selecting the music to some extent. Whether you call that policing or not is presentation more than fact.