It's seems to be peculiar to the folk scene, people trying to pass off other kinds of music as "folk", and calling people who obect "folk police". I think it quite unlikely that anybody would go to a rock'n'roll night with their choir and do a Purcell motet, and then object if someone said "Very nice, but's not very rock'n'roll". People might love it, but that wouldn't make it rock'n'roll, and it would not be considered a "fascist" or "police" comment if you pointed it out.
    I don't know why this is, there must be a psychological explanation. I play all sorts of music, but if I'm playing something that isn't folk I feel no wild urge to convince people that it is. Even if I strum a guitar while I sing it.
   I presume it is basically the inherent strength of actual folk music that generates this problem: people who love folk music feel a desparate desire that whatever music they make must be folk music too; and they can't bear it if some "Emperor's New Clothes" type boy says"But it ISNT folk".