Yes, the Protest Song continuum is another legitimate argument, and the Almanacs is an even earlier example than Dylan of new songs being called folk songs by very influential players. I think this is a US thing, where history is seen differently to the UK, with US influence on the UK folk scene being the force majeure.

Crow Sister, I'm sure some of our resident lawyers will correct me when I fail to say 'quasi-legal' every time. 'Trad' is accepted by PRS and other licencing bodies as meaning PD. This is not statute law, but a legal precedent in civil law, which is almost as good.

That said, if an individual falsely claims ownership of a Trad work, the claim is (eventually) overruled. If an individual falsely claims a copyright work is Trad it will, however, probably stand - so its not fool-proof.

Also, there are plenty of 'Trad,' Public Domain works with known writers. Plenty of 'trad' works still in copyright (specially tunes). Plenty of unofficial 'creative commons' works. Plenty of 'anon' works which are not 'trad.' And so it goes.

But not very many still uncollected 'heritage' songs.