"Sadly, nobody has the copyright on the word folk"
Probably the least convincing and irrational argument of all times
Language is what it is because of usage and track record - it's how we communicate.
If I bought a tin labeled 'salmon' and go home to find it was full of butterbeans because John Wests had decided salmon wasn't making enough profit, I would be entitled to go back and complain and, if doing so had caused me time and expense, some recompense (after all, the word "salmon" isn't copyrighted
If they persisted, I would be entitled to sue them.
The misuse of existing words is usually down to ignorance and often stupidity.
The deliberate use of words in order to push your own interest or product is simply dishonest and often dealt with through the Trades Description Act.
If you have something to give or sell you are morally obliged to describe accurately what it is.
Your argument is simply dishonest cultural vandalism - "I will call my music "folk music" because it suits me to do so - it doesn't matter how it affects the recipient.
Jim Carroll