It's not the first order effect (ie. keep bad guys away) that is the point with restrictions like these. It's the second order effects.

The thinking goes like this: If all non-terrorists obey the 'no-fly-zone', then obviously anyone violating it is a terrorist and can be shot down with impunity. Ergo, an increase in security.

Don't bother pointing out the flaws with this approach, I know what they are, I was just explaining that the supposed security is an indirect effect, mot a direct one.