Here's my 2p for the theory.. the 737 has rotation on main gear controlled by the shimmy dampers fitted to the end of the torque arm pivot pins. If you release both outer brakes, you would get rotation in opposite directions and therefore toe on the main gears and accelerated wear and stress on the legs. By braking the wheels on the same side of the leg each side, both will rotate the same way and this will allow lateral movement of the whole aircraft reducing stress on the main legs. It's only a few degrees but there you go.