Nothing except for the fact that the veto is not absolute. The veto may or may not survive his term if another president and another congress decide to pass and sign the same bill. As to your view of the "oath". If Bush or any president is in violation of their oath because they feel a bill is unconstitutional and signs it anyway then you can't stop there. If any president enforces existing laws such as the Brady Bill when they are on record as saying it is an infringement on the 2nd amendment then they are also in violation of their oath.
If a President fails to enforce a law because he thinks it's may be unconstitional, then if the law in question was constititional he has just violated his oath of office. While you are correct that a President is duty-bound to refuse enforcement of statutes he knows for a certainty to be unconstitutional, a President is generally expected to yield such determinations to the Court.
By contrast, however, a President may freely veto legislation because he thinks it might be unconstititional, or because the copy before him is printed in an annoying typeface, or for any other reason, without risking violation of his oath of office. Because he is under no obligation to sign legislation, he is duty-bound to veto any legislation he believes is likely contrary to the Constitition.