"I'm not sure which 'real folk musicians in Europe' you're referring to" Actual folk musicians of the old-time era, on both sides of the ocean, used scales that didn't supposedly fit "well" with the chords they used; that wasn't seen as an issue. When Dave Van Ronk sang a flatted third over I (which "clashes") because John Hurt or whoever had done it, that sort of thing, he was making folk music by emulating real folk music, not just doing whatever he felt like and calling it "folk" music.