The Fourth Amendment prohibition against illegal search and seizure made it into the U.S. Constitution precisely because its drafters understood that the power to snoop is addictive. Even if we happen to trust in the good intentions of the snoopers, the nature of any government can change rapidly — which is why the Constitution places limits on the tools available to any regime. But the drafters could never have imagined the commercial pressures at play today. The global homeland-security business is now worth an estimated $200 billion — more than Hollywood and the music industry combined. Any sector of that size inevitably takes on its own momentum. New markets must be found — which, in the Big Brother business, means an endless procession of new enemies and new emergencies: crime, immigration, terrorism.