Seems like a common sense measure to prevent such items from getting close enough to the magnetic field to be a threat.
I believe that metal detectors work by using magnetic fields: These things put out such overwhelming fields, my guess is that it would be useless anywhere nearby. I've seen cases where the monitor screens of the office computers used by the staff were skewed, and this is a good distance from the imaging hardware and behind well shielded walls.
Mark
it will also erase the magnetic strip on your credit cards and drivers license