Lockdown as a security module

I agree. The person holding the device signing keys is the de facto owner of the device—as in the one with the ability to exercise the rights of the owner with respect to the device. This is much more relevant than whoever the de jure owner might be. I would go so far as to say that claiming to sell a device while withholding the device's signing keys and not transferring control to the new de jure owner is an example of fraud. The sale has not been completed until full control of the device has been transferred to the new owner.