It may have been their impression that all posts were considered adaptations, on which re-licensing to a newer version would indeed be legal. (This is why they processed dissociation requests on all posts, even though the 3.0 license only required it on adaptations, i.e. edited posts. From the signals I've picked up, they only do dissociation
because it's a formality required by the license, and that they'd prefer not to do it.)