Actually, the reason I placed my infants rear-facing in cars is because their chests (which would take the brunt of the force because that's where the restraint belts of the car seat are) can't take the force of deceleration - like in a panic stop, which is more common than rear-end collisions. So in a sudden stop, their backs (instead of their chests) press up against the back of the car seat.

I'm treated to roughly one aborted takeoff and/or hard braking landing each year, and my kids got to experience one in their car seats.

And the whole time my girls were rear-facing in the car, they were rear-facing on the airplane. Each and every time. There are no FAA rules that require that infant seats be placed forward facing. We managed to do it on ATRs and mainline jets. Not a lot of room on some airplanes, but plenty of room in coach bulkheads, and of course, J and F.