The only argument I can see for this is as follows: it's a well-known fact that Bush is trying to avoid his father's fate. To do so, he probably needs a growing economy. Having already promised in the 2000 election to sign this travesty of a bill (in order, I guess, to win open primaries against McCain), he may not want the gun issue to escalate into a huge fight right now, losing him the votes of RINO's he needs for his tax cut package.
If this is the case, and I'm hoping it is, he can reverse himself on this issue after the economy is growing nicely. He cannot risk angering gun owners, especially since many of them are union Democrats who support the minimum wage and oppose right-to-work laws, but who vote Republican for the gun issue alone. So make sure you contact the White House about this! Preferably with email and a written letter.