Copyright law where songs are concerned seems to have come to the fore at very much the same time as social singing died out - probably not coincidentally - but there are some overlaps. Jean Ritchie copyrighted "Nottamun Town", partly because she added two or three lines to it but mainly, I think, because if she hadn't somebody else would have done - and it 'belonged' to the Ritchie family at least as much as it belonged to anyone else. Whether that meant she was paid royalties on it I don't know, but I very much doubt Jean considered it to be her song in the sense that "Bridge over Troubled Water" is Paul Simon's. (Perhaps in the sense that "Scarborough Fair" is Paul Simon's...!)