The nature of folk music is not restrictive otherwise it becomes a shallow copy rather than a full-blown living representation of a folk culture. It sounds to me that these erstwhile Anglo-Saxon groups are using folk music as a synthetic veneer, not a vital tradition.
Music is a unifier. Unfortunately there were Nazi songs that were intended to unify some sick people.

Hitler used Wagner (who was an anti-semite) in this way. The myths were intended to bolster the supremacy view. It became a characterized cartoon of itself.

Bluegrass music runs the same danger in the US. I don't like to see the rebel Confederate battle flags flying at these events.

Association by ideology that promote racial supremacy with certain musics are destined to
be laughed at by future generations. The BNP are neo-Nazis that will not be important in
the annals of history.