I note that some of these tunes are identified as mixolydian that I thought were properly Dorian in scale. The whole-tone-down thing (G/F or D/C) is not at all restricted to mixo tunes. If the tune is pentatonic, you can have real problems identifying the difference, if, for example, the third is omitted. For autoharp players, as WYSIWYG is, tunes which are played on banjo in mountain minor or one of its variants (D tuning tuning with a raised 3d, for example -- f#DGAD or aDGAD) will often end up having this I/bVII chord change in 'em. The mixo tunes will sound okay with the I and flat VII both major, and the dori tunes will sound best with the I as a minor chord. An example would be using Am/G for the Coo-Coo Bird.

Bob Clayton