"Well ichkeria, it's because you can not see who is behind it. As the recent army (SAS ?) raid in IS held territory shows. No one has ever suggested that a ban will cure all security problems though. "
You can't see behind lots of things! Not sure it's fair to bring SAS raids in the Middle East into it. There are plenty of situations where head covering is inappropriate and should not be allowed: the trouble with trying to introduce a general ban of this sort is that you have to dress it up (so to speak) as something it is not, and you end up looking silly: France did not introduce their ban for security reasons of course. Their ban applies, as far as I know, to lots other types of head gear, and is effectively unenforceable. But the motivation behind it, and the reason it got through, is because of their idea of secularism.
Anyway, I daresay this has been debated ad nauseam enough times. Sandy's first answer was the most relevant to the question.