For the record, I don't think Apple does this entirely for security. I think they do it to stop the illegal distribution of items which leak from Apple's supply chain, as well as counterfeit goods sold as though equal in quality to Apple parts. (I know multiple people who have had non-genuine screen replacements and they've always been low quality fragile glass.)

If someone tried to sell whole iPhones assembled entirely from parts leaked from Apple's supply chain, it ought to seem obvious to anyone that this is wrong. Why is the principle any different when it's not an entire iPhone?