On trailers like most of us tow, we use a WD hitch, with the spring bars loaded, to help distribute the tongue weight. In that case, the safety chains are there to help control the trailer in case the hitch breaks or, as in the case above, the ball itself breaks. The tongue cannot come off the ball when using a WD hitch. That can only happen when towing a smaller trailer such as a boat or utility trailer that does not use WD.
This is the reason that the emergency brake cable MUST be connected to a place on the tow vehicle itself and not to the hitch. On my truck, there are frame loops under the rear bumper that are provided to hook the cable to.
Barney