I could also argue that it is simply unnecessary. Consider all of the security measures already in place in airports. We have bomb sniffing technology, x-ray machines, we remove our shoes, we are randomly searched and wanded, there are multiple identity checks, a limit on liquids, a long list of banned items and a no fly list. If this is not enough security to effectively make flights safe, what real difference could this new technology make?
The intrusion and potential for abuse of these kinds of x-ray machines is simply too great and the benefit for security too limited to be a wise move.
The only way to say that the US is not justified in doing this is to say that it is too great of an intrusion on people's privacy and that it is a search of a person without a warrant.
You can argue that using the body imaging technology that you have heard about is too invasive because it would be possible to see the details of a person's body under their clothes. You can argue that this is too intrusive of a thing to do if you have no specific reason to search that person.
To me, that is the only way you can argue that it is not justified.
In discussing this question, it is important to remember that acts of terrorism - which the full body screening and other security measures are intended to combat - hurts people as much by violent attacks on people and property, as by the fear it creates in the mind of the people, and the cost, inconvenience, and indignities people suffer in the process of preventing terrorism.
Most certainly people and countries need to exercise caution and care to prevent terrorist action. But the cure of a problem should not cause more harm than the problem itself. One important consideration in the fight against terrorism is the extent to which we use the defensive approach, which punishes honest citizens much more than the terrorists, and the extent to which it seek to identify and weaken the sources of terrorism.
In my opinion body scanning is justified if it helps in detection and elimination of sources of terrorism. But if it is jut one more technological step in our unending march towards increasing submission to fear of terrorism, then we really need to do some hard thinking on this issue. In the least, the proponents of installing this system should produce reliable figures on the cost of full body scanning in terms of infringement on privacy and hassle free move of honest people versus the benefits in form of prevention damages from possible terrorist attacks.
To best of my knowledge, there have been no serious terrorist attacks on American soil after the 9/11 incident, and terrorist attacks elsewhere in the world, in any case, would not have been prevented by security system limited to USA. I am not saying that USA does not face the threat of terrorism, or that it should not take rigorous measures to combat it. All that I am saying that it need to recognize that the fear psychosis created by terrorism also hurts people, and fight against terrorism should aim at reducing this kind of damage also.