Having the tube rotate with the wheels has several advantages.
First , it synchronizes drum speed and therefore clean off speed with the speed that the sweeper is being pushed.
Second, by having the direction of rotation match wheel direction of rotation, ensures that the drum can roll over any obstacles if it does hit the ground.
By cleaning the magnet off continuously, the length of time the magnet could pick up steel shot without stopping is only limited by the size of the bin in which the steel shot is being deposited. The size of the bin is also restricted by the size of the drum. Therefore the bin size was restricted to a size that would not overly exert the operator upon removal of a full bin of steel shot in normal conditions. And with the steel shot being deposited directly into a removable bin, there is no need for the operator to look for something to deposit the picked up steel shot into.