Well, they do. iPhones are still valuable after being parted out, although Apple has been combating this by requiring parts to be registered by them (and shamed for much of the same on HN)
Deservedly. If there was a normal market, or at least access to spare parts, there would be no need for donor devices.
The warehouses full of people in Shenzhen tearing down millions and millions of stolen iDevices would still be chugging along, inserting stolen parts into the supply chain.
Apple enforcing DRM on parts is very much a pro-consumer move, steadily taking us towards a world where a stolen iPhone will be worth nothing.