It's not so much the issue of the direction being faced relative to the motion of the aircraft, it's the ease of evacuating the infant after the aircraft has come to a stop.

A seat that faces the seatback is more likely to have it's restraint system obscured that one which is facing forward. That makes it easier to free the child from the safety seat, and carry the child to the nearest exit.

I suspect the FAA rule is more lax because of greater commercial pressure on regulators to make a car seat that is readily transferable from car to aircraft.