Strange as it may seem I wouldn't doubt if it's because of things like alternator capacity, cooling capability (engine and transmission) and even wiring harness? Usually a manufacture installs a larger capacity alternator, trans and engine cooling system, etc., when a tow package is ordered and without it the manufacture does not want to take on the liability should something go wrong. It's not really "extra" capacity with a towing package but capacity that equals what is demanded when towing. Doc I would bet that if you added all of the wattage demand of every electrical component on your car at full power, wipers at full speed, high beams, brake lights, running lights (or 4-way flashers) audio system, glove box and interior lights and I mean everything being on at once, the total would be pretty close to the total capacity of the alternator that was installed on the production line. Not less but very, very close to the total that could ever be demanded of the original alternator. That's a reason the manufacture might be saying it should not be towed with and if you do, and something goes wrong (little likelihood that something would IMO) then don't hold us liable. Their CTA (Cover Their A$$) way of putting on the minimum cost production alternator, cooling system, small capacity battery, wiring harness, etc.,.

Even back in the day they had towing packages for Mopars and for what ever reason you couldn't order it on the A12 package '69 B-bodies? An A12 440 IMO would have no problem towing but was it that it only came with a 4.10 ratio rear gear or standard with a smaller group 24 battery?

This one was funny to me but when I restored my '68 Road Runner I worked on trying to get my front bumper running/parking lights to stay on with the headlights like I remembered my back in '69 Road Runner parking lights did. After many attempts I realized the '68 RR had four side marker lights and the '69 had no side marker lights but reflectors. The standard low watt output alternator would not support the extra bumper parking lights, side marker lights and headlights so one or the other had to go with the headlights on......."strange as it may seem"