I think they need to use a little common sense at times!
Funny as it may sound, I suspect they are required
not to.
They are drilled to do a set sequence of actions, and must not deviate from that.
case in point - A friend of mine has a small pair of glass in a thin plastic case. He showed them (DXB security chimps) the specs and case, but they refused to let him carry them through the magnetic detector. They
had to go through the x-ray machine.
I too pondered this, and I suspect it's an arse-covering thing so if there's an incident on the plane they can rightly claim that they did all the right things, etc.