That's actually the most reasonable explanation I've seen so far, and it helps explain a bunch (one small correction: most states have a towing speed limit of either 55 or 65 mph, so just about the same or slower than in Europe. Canada is even worse with most provinces limiting highway towing speed to 80-90kmh).
That said, it still doesn't make any sense that our tow rating does not take into account the presence of a brake controller (that is to say, the stated towing capacity does not list braked and unbraked separately in most cars except trucks).
