content: You might not completely appreciate what the keys are for.  Apple doesn't want you, or anyone else, to be signing an OS to run on their phones, and withholding their private key is how they achieve that.  Additionally, if you can install your own keys to actually, y'know, sign the OS--then that means, suddenly, it's possible to social engineer people into installing new keys on their iPhones and then taking the devices over. All public key cryptography counts on every party involved keeping their own keys safe, which means that in order to securely verify that you're getting code from Apple, they have to make a private and public key, put the public key in the phone when it ships, and keep the private key secret--that's just normal cryptographic practice.