I don't get what everyone is having a hard time with here. Let me simplify by giving a list of actions listed in order of safety (from safest to most dangerous):
1) Flying in a car seat
2) Flying without a car seat
3) Driving in a car seat
4) Driving without a car seat
It stands to reason that the government would rather not ban #2 if it would force families to go to #3.
You don't hold your kids in a car. Why hold them on a plane over strapping them into a carseat?
Simply put, because holding your kid in the car is way more dangerous than holding them on a plane...