It depends. For example, there is nothing that technically prevents adding 'add my own key' functionality to the Android phone bootloaders - that would allow us

It depends. For example, there is nothing that technically prevents adding 'add my own key' functionality to the Android phone bootloaders - that would allow user to unlock bootloader, install AOSP or any Android build of their choice, and then lock bootloader again. The fact we have no such function in bootloader is not rooted in some technical tradeoff between free and secure.