This is a simple case of the routers adhering to the specs out of the box and that the spec could not be modified via software, and I really haven't seen that many TP Link routers that can actually broadcast outside of the "safe" spec, most of them can be unlocked to use the forbidden channels on 2.4 but their power limits are pretty low.
"TP-Link violated the Equipment Authorization and Marketing Rules by marketing routers in the United States that were shipped with TP-Link software that permitted the user to change the country code for the router, thereby enabling the router to operate at a higher power than allowed on certain restricted Wi-Fi channels."
This doesn't seem to have much to do with OSS/3rd party firmware support which is a whole other can of worms that the FCC opened a few months ago. This is also seem to be limited to 5ghz only where I guess the US has more restrictions.