I was only just finished saying on the fRoots forum (a propos of Jon Bowden's spirited defence against a below-the-belt attack on the Bellowheads' version Flash Company) that even new songs do kind of belong to everyone as soon as they are sung out, Dick, so I'm with you there.

But it's still only 'kind of.' Good manners still apply. I'd not change any song by a living writer without permission, or use a definitive arrangement of a trad song without permission either (though that's an area generally more grey).

As you say, this is a much more clear cut case. Open and shut in fact.