I was never under the impression it was the Undertaker's contract that limited his public appearances. I always thought it was the deadman gimmick that did. Him not making many public appearances for the WWE fits the gimmick. When he was the American badass it seemed like he did more appearances. As the deadman it just makes sense that he wouldn't be out promoting the WWE because it just doesn't fit the character at all.
Plus, from all the stuff I've heard and read about him, he has an old school mentality and wants to maintain some secrecy and intrigue to his character. He's done a great job of that over the years.