Doing what comes naturally, on purpose, to share with people who will do WITH it what comes naturally to them.
By extension, we could say that Mozart wrote folk music too, just from/for a different cultural outlook. For some, what comes natcherly is pretty complex. In the writing, playing, hearing, sharing.
I would guess that the folk music any individual can identify as "folk" tends to be that of our own culture or one to which we are attraced by commonality or by desire for divesity. At least it's like that for me.
Folk = people = my people = The People
As the Native Americans would speak of themselves, not as this tribe or that, but as The People, meaning (I guess) their people.
Look ma, I'm riding Klezmer. See you around the block!