Fair point - Stephen Foster comes to mind as well.

But I suppose, without in any way being a purist about this, what I'm talking about here is songs that have been accepted as 'traditional' to the point where it would be really surprising to hear of a named author. Somebody must have written Plains of Waterloo - just as somebody must have written Searching for Lambs or The Outlandish Knight - but having an author's name put to it would be a real jolt.