Musicmick, your historical view of folk music USA is completely accurate. Much of it was brought to the public by the influenced by the New Popular Front headed by CPUSA leader Earl Browder attempting to bring the culture of the "masses" into prominence.

It has to be said that "agit-prop"music has a venerable history in the U.S. even prior to Joe Hill when you find ideology in earlier forms of what we call folk music, the political ballads such as Yankee Doodle, Lilibulero, The World Upside Down and early protest music.

Calypso and Jamaican music can be classified as "agit-prop" as well.

Even Chopin wrote a "Revolutionary Etude". Beethoven had an axe to grind also.

You might be surprised to find that nursery rhymes were originally political statements such as "Little Jack Horner" (Archibald) and "Mary, Mary Quite Contrary" (Mary Queen of Scots).

A good song is a good song, agit prop or not, regardless of the motive for writing it.