To protect players. The defender stopped looking at the runner because he lowered his helmet. If the runner hadn’t then leaned into the shoulder and to the right then it would have been hitting with the crown, which the defender had no way of knowing since it happened after lowering his head toward the runner. The only way to get rid of the result, or the “hitting with the crown,” is to also get rid of the action, defenders lowering the crown toward a player so they can’t see them.