"...songs in the other types of folk music have no known composers..."

An old, old chesnut this - but it's just not true! ANY song can become a folk song - it just has to be selected by the 'folk'.

Here in the UK many songs selected by the 'folk' in the Nineteeth Century, and earlier, were published on broadsides (ie. printed song sheets offered for sale). The people who wrote the songs on the broadsides just didn't sign their work, that's all. Presumably it either didn't occur to them to do so, it wasn't the 'done thing' or the broadside publisher claimed ownership of the song once it was committed to print.