I call that a smart move.More than that, I think it's just a cheap cash grab. Not that he needs the money, but it's a cash cow he can milk year over year.
Paul was the sole writer, producer, and performer on the track, doing every part. I believe he owned his publishing rights by then too, so basically it's an easy novelty song guaranteed to get airplay every December. I just read that he collects approximately $400,000 per year on royalties for that song alone.