Every society accepts some risks as part of its overall social contract. People die when they drive cars, they die when they drink, they die from crime, they die when planes go down, they die on bikes. The only way to eliminate the risks would be to eliminate the activities -- no driving, no drinking, no weapons of any kind, no planes or bikes. While risk/reward tradeoffs vary between, say, Sweden and China, no nation accepts the total social controls that would be necessary to eliminate risk altogether.
Yet when it comes to dealing with terrorism, politicians know that they will not be judged on the basis of an "acceptable level of risk." They know that they can't even use that term when discussing the issue. ("Senator Flaccid thinks it's 'acceptable' for terrorists to blow up planes. On Election Day, show him that politicians who give in to terror are 'unacceptable' to us.") And they know for certain that if -- when -- a plane blows up with Americans aboard, then cable news, their political opponents, Congressional investigators, and everyone else will hunt down any person who ever said that any security measure should be relaxed.
This is the political tragedy of "security theater."