Are chainsaw helmets meant to protect you from the saw, or just falling debris?
The helmet is designed to protect you from falling objects. It is also the mounting point for the face shield and ear protection.  The helmet will also provide limited protection from the saw in the event the saw were to kick-back or the operator lost control and the chain were to impact the operators' head.  It will not provide protection from a running saw being held against the helmet. This differs from safety chaps which are designed to actually stop a saw in the event of contact with the chaps.