The point of copyrighting a traditional song, as I would think most people have grasped by now, is not to claim ownership so as you can get royalties off others who record it. You can't do that. The point is to get the royalties from your own recording of the song, which would otherwise be trousered by your record company if there is no copyright claimant. And since researching folk material, in my own experience, takes a lot longer than writing original stuff, I think the practise is perfectly legitimate.