I've always wondered about this. George W. Bush did it too: declare that a bill passed by Congress was unconstitutional, but sign it into law anyway. Doesn't the oath that a president swears to take office have him vow to preserve, protect, and defend the Constitution? I should think that would require him to veto any bill he believed was unconstitutional. Just more evidence that none of them (at least since Grover Cleveland, who actually did what I'm talking about here) really believe that the Constitution limits them.