Bush waited pretty late into his lame duck period to pull this particular stunt, so it seems this is mostly a favor to his successor. He wants John McCain, Clinton, or Obama to be in a position to commit widespread abuses and not just hog all the glory for himself.
To the extent that this is about his successor, my guess is that they figure that Congress will rediscover its interest in oversight and objections to presidential executive power overreach. The very powers Bush claimed will, for a Democratic president, be the foundation for impeachment. They aren't just masters of hypocrisy, they're masters of "distinctions without differences." That is, when President ClintonObama does it, it's somehow different when President BushMcCain does it. Don't worry, Cliff May will explain it to Wolf Blitzer and Pete Hoekstra will explain it to Joe Klein and it'll all make sense.