Actually, I would argue that this is not true. Before installing, the device wants a ticket to be signed by apple that contains a hash of the firmware to be ins

Actually, I would argue that this is not true. Before installing, the device wants a ticket to be signed by apple that contains a hash of the firmware to be installed, the phone's identifier and a nonce it has just generated. See here: https://www.theiphonewiki.com/wiki/SHSH https://www.theiphonewiki.com/wiki/SHSH So by not signing any requests for that particular firmware hash, Apple can effectively neuter that firmware and make sure it's never installed anywhere but on the target phone. The problem is though: If apple can be compelled to do this once, they can also be compelled to do this any other time.