[BJC]

The basic reason I can see them doing this is for enforability. It's already difficult to determine if the robot will fit within the 54" cylinder when on flat ground. Robots breaking the cylinder when transitioning between levels while climbing (especially unintentional) is basically impossible for the refrees to be certain of. In close matches no one wants the deciding call to be made by a ref who from 15 feet away may have seen a robot momentarily break an invisible cylinder...