It's not. That's what the Attorney General is supposed to do. The SCOTUS will not give advice, since they only rule on cases. I think the AG or Solictor General could appeal to have the law overturned, but if you are going to take the heat for that, why not just veto the bill? They clearly do not have enough votes for an overide.
The result is the parts good for GOP are passed, the parts that are not constitutional are not enforced and the Rats learn not to mess with Bush.
I used to think Bush had to veto it to avoid the "no new taxes" hammer, but I now see another way that would work even better.