Actually, if you look at the inside of the skull, you will see a very rough surface. There is not enough CSFluid to protect the brain from grating against this rough surface during a strong rotational force. The force one would exert during a simple fast right and left is unlikely to cause damage. The rotational acceleration would need to be faster than most neck muscles can generate. Repeatedly whipping the head back and forth is another issue. The cumulative effect of this repetition could cause trauma.