Tow specs- what's behind them?

...and yet another prospective owner has posted a "will my minivan tow a TM" question. I wrote one myself when I was buying.

We spend so much effort trying keep within the tow specs of our TVs and even building in large margins, but, at least for the minivans and crossover SUVs, where do these specs come from?
- Why is it always a round number (yes, there are exceptions, but they are few)?
- Why is it always the same number (3500lbs/350) no matter what the manufacturer?
- Why has it stayed the same at 3500lbs as the models have gotten more and more powerful engines over the years?
but most of all:
- Why, where the same vehicle is available in Europe, Asia, Australia or NZ ( sometimes with smaller engines) is the tow capacity 4400lbs there ( i.e. 2000kg -still a nice round number) instead of 3500 lbs???
Are they really all reckless idiots over there, while we are not? Is it because we Americans are so big and fat that the typical case has to include an extra 900lbs of margin. Is it something in the air??

Well I don't know, but it's probably due to a qualifying emissions test or a trade regulation or litigation margins or something else not particularly driving/towing related. Does anyone know?
We treat the specs like a magic number, when they're obviously not. However, the specs are all we're given to work with so we keep to them, but I can't help being skeptical.

-Paul