Interesting thread. Of course most of these mistaken ideas about where a song comes from arise from the fact that most people cannot be bothered to look to see who wrote the song in the first place.
Then of course songs become 'adopted' by the people who sing them. Songs develop a life of their own as they make their way around the world. To insist that Dirty Old Town is 'about' Salford ( or whatever other north of England town you think it is )is rather pedantic. It has become a song about any dirty old town.
As to the 'tradtional' songs by mr and mrs anon, these songs and tunes have found their own ways around the world and will continue to do so. Each may claim them for their own but they can never 'own' them.