They do hold in reverse "somewhat" but not near as good forward, simply because the way brake shoes are made. The short shoe (leading) on the front and long shoe (trailing) on the rear, so they naturally hold about twice as strong forward.
Actually, GM addressed this issue in the early 80's by putting the cable in thru the back section of the backing-plate to have the rear shoe become the leading shoe, and reverse rotation then applies the front shoe .