Guthrie published the song in 1945 (although he wrote it five years earlier). At that time copyright extended for 28 years beyond the date of publication, after which it could be re-registered for a further 28 years. Guthrie did not renew the copyright, and so it should have entered the public domain in 1973. A publishing company, though, registered the song as a new creation in 1956 (eleven years after Guthrie published the song) and renewed it in 1984 - by which time the length of copyright had been extended considerably. Clearly (as with Irving Mills and "St. James Infirmary"), they had no right to ownership of the song.
Guthrie based his melody on earlier songs.
An old turn of the 20th century Baptist hymn called "Oh My Loving Brother":
Which The Carter Family used for their song "Little Darling Pal of Mine," recorded in 1928:
And again for "When the World's on Fire," recorded in 1930:
In this context it is interesting that - as I discuss in "I Went Down to St. James Infirmary" - song publisher Ralph Peer asked the Carters to modify the traditional songs they heard in their native Appalachia in such a way as to allow the songs to move from the public domain into copyrightable material. Peer then assumed the copyright for his publishing company, and kept the Carters loyal to him by assigning them a portion of the royalties (which was a better deal than most publishers were offering at the time).
Some writers, such as Barry Mazor in his important (if hagiographic) 2015 biography "Ralph Peer and the Making of Popular Roots Music," (Chicago Review Press - with a co-copyright credit to Peer's publishing company Southern Music) assert that this is just good business. The reasoning goes that it is the business of song publishing, and the profits that flow from it, that allow these songs to survive and enter public consciousness. In this way capitalism is good for our commonality and for cultural well-being.
More idealistic assertions suggest that song ownership should always reside with the writer, that simply because you have more money does not give you the right to profit excessively from somebody else's work; publishing revenue should be enough. Simply because there is a common practice does not make the practice right.