But the certification of baseband radios in multiple countries requires the devices to be restricted to operating within the parameters permitted under the regu

But the certification of baseband radios in multiple countries requires the devices to be restricted to operating within the parameters permitted under the regulatory requirements for the use of spectrum in those respective countries. These parameters are enforced in the firmware on the device. The manufacturer has to ensure that the device complies in order to obtain certification for use in a consumer product, and they do this by locking the firmware. If it's unlock-able, it may be perfectly legal as an uncertified device for use in a lab environment, but it isn't as a consumer device.