This is security through obscurity. To quote Bruce Schneier:

Security is both a reality and a feeling. The reality of security is mathematical, based on the probability of different risks and the effectiveness of different countermeasure... Security is also a feeling, based on individual psychological reactions to both the risks and the countermeasures. And the two things are different: You can be secure even though you don't feel secure, and you can feel secure even though you're not really secure."

The body scanners give the reactionary populous a reason to "feel" secure; even though the chances of a terrorist being caught by body scanners are slim. (see http://www.schneier.com/blog/archives/2010/01/german_tv_on_t...)