Well, ignoring the BS of this Administration about signing instruments and the monolithic Executive or whatever the hell they called it, in point of fact there is some valid reasoning behind the Bush claim, albeit (as has been common with this Administration) a precedent stretched into a brand new shape.
Yes, it is the privilege and prerogative of the Supreme Court to pronounce with finality on the constitutionality of any law. However, they are far from the only people capable of evaluating that and making judgements regarding it. Every court in the U.S., from the West Tumbleweed Plains Police Court to the top state courts and the U.S. Circuit Courts, may be called on to render that decision.
But further, the other two branches may be called on to do so as well. The late Sen. Sam Ervin of Watergate fame used to make a point of having his committee review bills intended to create new statutes to assure they were within Constitutional bounds based on the law of the land at the time. And the President can , and past Presidents sometimes have, refused to carry out laws they deem to be contrary to the Constitution. This is their right and privilege in carrying out their oath of office.
In fact, it may be not only the right but the responsibility of a citizen to disobey a law he considers unconstitutional. If, for example, I were a restaurateur in the South in the Sixties, with an experienced and skilled black waitress on my staff, and the state legislature made it illegal to employ a black person to serve foods to white people, it would be my obligation to keep her in my employ under the Fourteenth Amendment and the Civil Rights Acts passed pursuant to it, and ignore the unconstitutional state statute.
Further, “signing memoranda” are a legitimate part of the legislative history of a law, just as the contents of a hearing on the law are – used by the court to construe how a law is intended to be applied by the three bodies: Senate House and President, which together enact that law. They don’t constrain the court but are legitimate information to take into account.
This is not saying that Mr. Bush is justified in how he’s dealing with the law, or that his mode of rewriting a law by those memoranda is anything but execrable. But it does furnish some valid light on what he and Gonzales may think they’re doing.