"Folk" as an adjective is almost completely worthless. Take any phrase that has it and remove the word "folk" and you'll find that the phrase still means exactly the same thing, without the excess verbiage. People only add it because they've fetishized the word; for them in connotes some extra-special-good quality, one that really doesn't exist, but one which they and others with the same fetish have convinced themselves exists. When they use the word, they are communicating to those "others" that they share the sentiment/fantasy; it helps them identify one another, puts them in the same club.

Q: "What is a folk song?"
A. It's a song.

The characteristics of a song --how it was learned, who wrote it, what is its intent, how fast is it, what instruments is it played on, what gender performs it, etc etc etc-- are all different and notable. But once you've slapped it with the "folk" label, you've stopped saying anything objective about the song and started saying how you feel it fits the fantasies of a particular set of people.

Gibb