Also, some of them have become so well-known that, whether the author likes it or not, they've passed into de facto public domain. I keep running into Shoals of Herring described as a 'traditional' song.

A lot of folksongs, even the published and authored ones, have undergone a great deal of change, passed into cliches or become local staples. Waltzing Matilda and Flower of Scotland are "unofficial" national anthems, for example.