once again the pulisher nazis strike. folk music is flok music. how songs in this database bemoan man's pretense to own land, mountains, sea shores, etc? how much more absurd is one man or woman's claim to ownership of performance art material.
as a songwriter and owner of copyrights, I understand the artist's wish to garner monetary recompense for his or her art work, and this opportunity is pretty well protected in our modern society ... but this demand from NMPA is truly exploitation.
you are right-on in pressing them for specifics. if they cannot provide you with specific complaints, they cannot expect a response. I would simply tell them to piss off, but then I am a confrontational type.
best of luck with these snakes ....