Actually, it is the case. Just because a law is null and void, however, doesn't automatically prevent law enforcement or the courts from acting on it in an unconstitutional manner.
Our oaths of office enter the picture at many points, including legislation, law enforcement, and the judiciary. Adherence to one's oath of office at any point would render the law null and void.
When I, as a U.S. Citizen, lawfully choose to ignore an un-Constitutional law, it is immediately rendered null and void throughout my sphere of influence. If higher authority should screw the pooch, it does not impart any Constitutional legitimacy to that law. That un-Constitutional law remains un-Constitutional, even if the Supreme Court itself declares otherwise.
People, I think you're forgetting the checks and balances built into the system. Justices of the Supreme Court are only allowed to serve so long as they exhibit good behavior. Do you think they'll be allowed to continue serving if they toss the Constitution out the window? No. One check/balance against that is Congress, who has the authority to remove them. A second check/balance is We the People, who have the authority to remove members of Congress who fail to do their duty to removing them. Our Founding Fathers themselves mentioned additional checks and balances associated with their recognition and protection of our right to keep and bear arms.