@Chippy_Tea, don't disagree with you re. probability of bolt shear (it was just an example). Though failure through fatigue rather than load is more likely.
But there are other modes of failure. I've witnessed carbon seat post failure (without tagalong) which was pretty spectacular on a trail, nevermind on a road.
Manufacturers are keen to divorce themselves from liability, with detailed instructions prescribing set up, maintenance and use. Many riders ignore the caveats, happy to risk all on poorly maintained bikes.
Only this last week I helped a kid refit his pedal which had fallen off mid ride. Had it been in traffic, it could have been a nightmare!
Each to their own, but I'd personally prefer not to subject the survival of my entire family to a single point of mechanical failure.
Incidentally, Trail-a-bike state...
Recommendations:
• We recommend using only a high quality chromoly seat post to tow the
Trail-A-Bike. Seat posts made of thin alloy, steel, or carbon fiber, could
bend or break.
I'll bet most riders haven't given it a consideration.