Think about going uphill and the trailer detaching and running under a public bus or something. The securing cable does not necessarily need to be from the hitch manufacturer - just any reasonably thick cable with loop ends made.
Driver's are supposed to take evasive actions based on their own safety. Of course, they might not necessarily be able to instantly discern whether you have a kid or cargo onboard, but the bus could just go right over the top of a cargo trailer and keep going.
While getting something caught in the rear spokes is less dangerous than getting it caught in the front, it could still potentially take a cyclist down, which could equally have dangerous consequences.