Actually, where the free leg is concerned, the situation is almost exactly the same for a backward walk, just in reverse. Same curved path, same fulcrum (the hip), same effort (the knee, not the foot). The only real difference is that it's swinging in the opposite direction. But it's still swinging.
On the forward walk as well as the back, the supporting leg causes the body to move through space. When the fulcrum itself moves through space in the same direction as that of the swing, it tends to increase the impression to the observer that the object is swinging. This could explain why people describe even fairly bound leg actions as a swing, when you consider that the legs are usually attached to a body that itself is moving through space.
Regards,
Jonathan