Part of the strategy involves constantly changing the conditions of security. The more predictable the conditions are, the easier they are to counter.
Of course in doing this, the conditions being enforced appear to be arbitrary, and in many ways they are.
There is no definititve solution; certainly not one that can be both 100% effective while simultaneously appeasing everyone who felt inconvenienced or violated by the implemented security measures.
But, as it's already been pointed out, measures, regardless of how intrusive, arbitrary and inconvenient they may be, are still a part of the job virtually everyone will hold the government responsible for.
If anything happens due to security measures being "too lax" or just plain non-existent, you can bet everyone who felt violated, inconvenienced or even just plain annoyed at having to take off their shoes while waiting at a security checkpoint, will immediately look for a place to point the finger at for failing to do enough.
At any rate, the scanners are supposed to make security checks more effective as well as more expedient (read: convenient). Hardly anything worse than being required to go through a metal detector or having a security guard sweep you with the wand.
