Bootloader enforcement mechanism

[PsYk0n4uT]

Can anyone explain the mechanism that is used in these devices that keeps us from downgrading the bootloader and/or booting unsigned images?
Speaking of the usa snapdragon devices specifically but I'm sure that all the snapdragon devices probably have the same setup.
I'm just wondering why it's not possible to bypass this via a hardware mod if we are never going to be able to soft mod them to accept our modified firmware

 

[PsYk0n4uT]

Also. I'm sure this has been covered as well. But is there not any way to flash the snapdragon bootloader and rom from the devices outside the USA? Feom what I understand those snapdragon devices are bootloader unlockable. Please correct me if I'm wrong.
What would stop us from flashing those firmwares?