Interesting the way the term modal is tossed around like it means something.

More correctly a lot of folk tunes do not use the 7 traditional diatonic pitches. Many use only 5 or 6 pitches. Tunes using 5 are called pentatonic (Amazing Grace) & this is very common. These tunes have a sound not quite major, not quite minor, so modal is what they end up being called.