So he's just being a cheap political hack, instead of screwing up some actual courage and principles and telling the gun-grabbers to shove it up their McGreeveys because it's an irretrievably bad idea.
Honestly, if Bush knew the AWB had a chance of passing in Congress he would have said he'd veto it. Look, you have to remember that a lot of people are very uneducated and uninformed when it comes to guns. These people think assault weapons literally means heavy automatic machine-guns and weaponry used in actual combat, when we all know it also applies to ordinary shotguns and rifles used by law-abiding gun owners. So by saying he'll "sign" the AWB, Bush is simply smoothing over those people's fears while at the same time neutralizing the gun grabbers.