If the vehicle is "LOCKED" for all units, then the reason can be only one. This means, that all doors is closed.
If all doors are closed, then any soldier not should have the access to the inside of this vehicle. How the player can get grenades, ammunition and even a launcher, if all doors of vehicle is closed? In the real life, this is not possible.
However, in the A3 we see the opposite. The player, always can use the inventory of a closed vehicles. This is a nonsense, please close inventory of all locked vehicles/