I think the comment about liability driving the tow ratings is right on the money. In our victim mentality here in America, we always reserve the right to make somebody else pay for our own stupidity. So, the ratings reflect a max tow load that the manufacturer is reasonably certain will protect them from liability. I has nothing whatsoever to do with the physical capacity of the vehicle.
It is also important to note, however, that not only the rating is dictated by litigation. So is your insurance coverage, and your personal liability in case of an accident. If I am driving a Dodge Cummins Turbo Diesel Dually that by some fluke is only rated to tow 500 lbs, and have an accident towing a 950 Scamp
Std, my insurance company will deny the claim quicker than a heartbeat, and I'll be left holding the bag.
So, Ken, your PT will tow a 13 just fine and perfectly safely, but the risks are WAY too high in case of accident. Don't do it.