you don't get the distinction jim.

what mick jagger does with the blues is not folk music.

what the artists in folk clubs do with mick jaggers music turns it back into folk music.

they are not working to blue print or the dots written down. the legal position is quite irrelevant. you could probably find a tune similar to most pop songs. where there's a hit -theres a writ -as the saying goes. but that's not the point in question.

does dave burland still segue rocking the cradle that's none of my ain/segue into my baby used to stay out all night long?
its when we take ownership and inhabit the song, squatters rights if you like, that's when it becomes folk music.