It arose as I recall from pushback from the big three about not holding the (higher profit) trucks to the higher standards, and the above loophole was the lobbyists workaround. Could it have been done? I don't know, but heavy trucks in Europe and Japan do seem to have better mileage for the same cargo capacity, but with the overseas cost of fuel in those locations, I think that one can understand the market forces there that stress fuel economy. IIRC, about the same time, there was a parallel discussion about increasing the fuel road taxes to bring the cost of gasoline and diesel closer to European pricing (currently $7+/$6+/US gallon). That went over like the proverbial lead balloon, and here we are.
I would point out that those standards have helped bring along things like more efficient automatic transmissions with more than three gears, and fuel injection. I for one do not miss the older vehicles' engines and fuel economy, nor futzing around with carbs and timing.
All the best,
Peter