There can be a bit of a rebellious, even subversive streak in folk music

We all know that, Steve, with the emphasis on can. Some ethnicities have the feeling that their culture is being oppressed, so that merely performing it amounts to an act of opposition - not necessarily left-wing, though. Also, it is always attractive for rebels to make use of folk-like songwriting - again not necessarily left-wing ones.

Still, most traditional music cultures of the world form strong alliances with their establishment, so that they are functionally conservative even if their governments claims to be left-wing.