... wat? No, I didn't. It's easy to imagine I "must have", but in fact there's no signature, no negotiation nor in many cases any consideration.
Ah, but you say: I must have signed a contract to use the app store that I downloaded the app from, and that must constrain me to honor the terms of the app that I downloaded, which is constrained by Apple's contract with the developer.
Except, no, I didn't do that either. The whole thing is a house of cards. There is absolutely no principle behind this regime, it's just something we've all come to accept because it's technically possible and because "usually" the power granted to hardware vendors hasn't been abused.
But it has bad side effects too, and it's really important that we as a community not lose sight of the fact that locked down devices are really, really bad.