Rights need constitutional protection because governments have a tendency to ignore them. But it takes more than a parchment dictate to force the authorities to do what they are supposed to do. It takes penalties that make violations painful to the violator.

That's where the exclusionary rule comes in. Police face constant temptations to break the rules to nab villains and put them in jail. But this rule goes far to discourage them. Violate a criminal's rights, and he may go unpunished.

That wasn't always the case. In the old days, cops paid little attention to the Fourth Amendment because they had nothing to gain by respecting it. Only when they had to pay a price for overstepping did they develop a sudden interest in staying on the right side of the constitutional line -- to the benefit of the innocent as well as the guilty. Wonder of wonders, police departments actually began training officers in how to comply with the Fourth Amendment.

After the Supreme Court imposed the rule on states and cities in 1961, New York's deputy police commissioner acknowledged as much. "Before this, nobody bothered to take out search warrants," he said. "[T]he feeling was, why bother?"

Anyone in charge of maintaining databases of arrest warrants may now be asking the same question. In fact, the court's decision creates an incentive for poor record-keeping. If Herring's warrant had been purged promptly, after all, he could not have been searched, and he would have gotten away with his offenses.

That may seem too high a price to pay. But the real objection is not to the exclusionary rule -- it's to the Fourth Amendment, since if his rights had not been violated, he would have gone free. The tradeoff between catching criminals and protecting privacy is not a daffy invention of activist liberal judges. It was the framers' whole point.

But the Supreme Court says it's fine for the government to profit when it deprives citizens of their constitutional rights through incompetence or sloth. That's like telling teenagers it's OK to get pregnant, as long as it's not on purpose.