I believe that is to prevent a vehicle from instantly turning 90 degrees at the beginning of its turn. Letting the player decide facing at the end of the turn helps with planning movement for next turn and makes sense because the vehicle is supposed to still be in motion, even after the driver ends the movement for that turn.
I agree that it needs clarification. I think this is another case of a word having multiple meanings. "Facing" is the status of the vehicle, but is also a passive status during the movement itself. The driver can drive in a zigzag pattern and while they are technically changing the facing every turn they make while moving, the Game mechanics only care about Facing when "stopped" at the end of the turn.