The best I can tell the pressure spike is steep enough that by the time the bolt releases the primer has already been blown. With the pin staying protruded through the face of the bolt through this period there is no longer a hole in the face of the bolt to blow part of the primer into.
Again this is my best guess. I have no way of verifying pressure curves in mine or anyone else's rifle.
I thought the spring on the firing pin in the original AR10 design was to hold the firing pin back, to keep it from potentially slamfiring or firing when dropped. I didn't think the spring pushed the pin forward. Wouldn't that be asking for slamfires?