The issue is starting from the boot-rom and up apple is effectively using cryptography to retain the property right of exclusion when it comes to what code can run on that ARM cpu. Effectively you have to ask apple for approval of any code that will run on your own hardware. If they sign the code they allow it run if they don’t its excluded from running. If you have to keep asking a 3rd party to do something with something you bought you don't fully own it.
The thing is normally a seller wishing to retain some rights to real or tangible property would normally require a legal contract between the buyer. Here however apple is using cryptography to bypass traditional legal means. You can buy an iPhone, iPad without signing any contract. Yet this effectively retained control of the hardware still exists. You can’t just buy the hardware and throw linux or android on it short of finding a weakness in how code is loaded.