Let's be honest and accept it's just about money and control to the makers of these devices. They will bring reasons such as these in their defense, but really they just want to keep the user from being in control of their device and e.g. making tracking them more difficult. I mean there have probably been tens of thousands of roms flashed on different devices. How many of them have emitted unhealthy levels of RF? And parents could retain the ability to flash their devices without giving that ability to their children. The reasons really just don't hold any water imo.