I think I and they rock their bikes because its natural and here is my well thought out reason why: when out of the saddle we keep our bodies still and rock the bike because if you want to keep your bike still then you are moving your body and wasting energy an losing aerodynamic form. More specifically when you rock the bike it is due to pedal stroke. You rock the bike because it gets your leg closer to the power stroke faster and more efficiently. If you were keeping your bike still when out of the saddle then you would be bouncing your bum, torso, and head up and down to get your feet to the top of the power stroke. Also if your moving your body then you are not able to maintain a constant aerodynamic position which is made more possible by moving the bike from side to side and keeping your body/ torso still.
I just pulled this out my bum but I am 99.9% sure of this. Now off to watch sprinting videos to see that I AM right.