I think Craig has fully covered his own self in case of accident, if it results from a loose container, BUT...
I ain't no lawyer,,,
I bet if someone gets hurt from a loose cart slipping away from the forks, your boss will get to talk to one.

What is unsafe and what is "ilegal" can be difficult to point out to people that may not want to hear what you are saying, and those words have different definitions to different people, I would urge a great deal of diplomacy in just how this point is made.

Unless someone ever gets hurt, or there is damage to the building or machinery, then no one can claim any damage from the way they are doing things.

I know OSHA can move so slow that a snail wold look like it's last name was Earnhardt, in comparison.

Read your operators manual, if it says you can drop it off a cliff, then you can expect to drop it off a cliff,, if it does NOT -mention- cliffs, then you can safely assume it was NOT designed with cliff dropping as a specification.

Ask yourself, and see if you can politely mention this to your boss, that what salesman say, may not be the exact whole truth.

DuoDelux said "then I'm not sure why they are supplied with a tow pin" they are equipped with a pin to be able to transport the truck. If the operator manual says it was designed to tow, then by all means tow.
I would ask you both to visit the "itsdf dot org" website, (the Industrial Truck Standard Development foundation) and download (and maybe read) the standards.

29CFR1910.178 "Only loads within the rated capacity of the truck shall be handled."
Seems pretty clear to me, what part of "within" is not being understood?
You ask; "Why are there forklifts only designed for lifting and not pushing or pulling?"
Forklifts are specially designed for a special job. Bulldozers are also specially designed for a special job. Tugs are also specially designed for a special job. Often the same company sells all those types pf vehicles, but gives considerably different instructions about their different operational abilities.
Bulldozers push, tugs pull, forklifts (or lift trucks) pick up and carry.