When a spring in a shock is compressed two things happen; the spring twists and the spring cocks. When this happens, the spring needs the freedom to do this otherwise the restriction results in mechanical friction in the shock, slowing it down in an uncontrolled and unpredictable way.
In motorsport, you would use a hydraulic spring pertch to provide a platform than can rock and twist isolated from the damper. Bicycles haven’t progressed to the point of allowing for this very expensive solution. A good example of the state of the art there is this HyperCoils part: http://www.hypercoils.com/spring-perches.html
Instead, at the very least, I provide a way to allow the spring to twist freely by making delrin washers for each end of the spring. It doesn’t solve for the rocking but at least it’s something.