Modes are theoretical systems that composers have used as a framework for writing--and there are many different   modal systems, each with their own sets of rules(and, often, the rules are not followed anyway)--folk and traditional music, even when they use scales that have modal characteristics, tend not to follow any set of formal musical rules--

The reason that the "Modal" names are used at all is that ethnomusicologists found them convenient ways to categorize the different sorts of scales that they found in the "field"--their sources generally had no idea what they were talking about when they said "Mixolydian" or "Dorian"--and anyway, even in Western European and American folk music, there were way more different scales than they had names for--

Certain folk instruments are, by their nature, modal, such as the Appalachian dulcimer and all the different sorts of pipes, and they have a logic all their own--and of course, you can play the tunes "modally" or not--My favorite example of this is "The Green Hills of Tyrol/Scottish Soldier". Just listen to any pipe band recording, then listen to the old Andy Stewart popular recording--