You end up forcing people to scale a fence, then specifically navigate through one of two metal detectors, which requires some level of precision. This is why it was probably deemed not allowed seeing as in a combat situation someone would probably want to retreat as fast as possible. It ends up to a very vague degree directing a player's movement. The same example can be found on the office staircase, metal detectors aren't allowed to be placed on the stairs themselves as they direct people's movement when going up and down.
Edit: The reason it directs movement is because people get stuck on the metal detectors when trying to get past them in a hurry.
