Er...they don't, really.
There are private terminals at every runway and passengers and planes are never searched at these terminals.
You could learn how to fly a Cesna, rent one, load it up with 500lbs of high explosive, and then fly it 5 minutes from JFK to Yankee Stadium and crash it during a game. BAM. 50k people dead, and you don't even have to file a flightplan with the FCC.
Tell me, how is that an effective deterent?
As I've stated 100s of times, the goal of terrorists is not death count. It's TERROR...it's to change our way of life. And at that, we have let them succeed past their wildest hopes.