I think it's probably so that they "retain" as much control as possible over their supply throughout the world. I remember talking to someone once about how Porsche is overzealous about protecting their secrets. Case in point...they use those one-use only screws on the GT3 Cup engines so that they can't be taken apart and "researched" and then reassembled. Maybe that's the reason why they do that with the 918 parts. Just a thought.