It seems ridiculous, but the logic is the same.
It's really, really good to have been trained in its use.
So you are standing there holding the thing up in the air with about lebenteen gazillion pounds of lead slung around your body, and maybe somebody to help you feed the belt up over your shoulders and you start firing IT WILL ROTATE.
Depending to some degree on your strength and weight it will rotate UP or it will rotate DOWN but it will rotate, and that first time it will rotate in ways you didn't imagine.
Grenades are safer for untrained personnel to use ~ which is why I don't want the government to issue you a machinegun. On the other hand I'm qualified and may have a use for it so I want mine!
Frankly the laws regarding machineguns are wrong. It's a self-regulating device, and your heavier machineguns, which are the best ones, really do require a support team.
The light machineguns, which are nearly useless except for building guard duty, were the ones the restrictive laws were aimed at. With higher powered ammunition, better rifles, better targeting systems ~ a guy with a semi-automatic probably has much real firepower as a light machinegun from the WWI era.