I agree that they could, but it should be my choice to pay the extra amount that it would cost at the time of purchase.
The government mandating safety / mileage features have priced new cars out of the reach of many people. What do they do? Buy older crappy cars that don't have these features anyway.
Go to the poor side of town and you will see that it is filled with mosquito foggers with forged inspection stickers.
My main point is that speed limits should be set based on safety concerns alone and leave the politics out.