From Marbury v. Madison: "So if a law be in opposition to the constitution; if both the law and the constitution apply to a particular case, so that the court must either decide that case conformably to the law, disregarding the constitution; or conformably to the constitution, disregarding the law; the court must determine which of these conflicting rules governs the case. This is of the very essence of judicial duty. . . "
If the Constitution is the Supreme Law of the Land, which one to uphold is a foregone conclusion.
It was mechanical. Another example would be if the Congress passed a law allowing the President to serve 20 more years. Constitutional conflict on the mechanics of government.
Conversely, you have super-legislative, where you have Justice O'Connor ruling that Affirmative Action is Constitutional today, but may not be in the future if its no longer necessary. Necessity being defined by her. Her ruling had nothing to do with protecting the Constitution, but rather protecting a law from the Constitution. The developing trend to use foreign law and mandate legislative action, makes my case all the stronger.
If it were intended for the SC to have veto power, the Constitution would have required bills to go there for review before becoming law.