As your link says, they intend to prohibit selling hardware which can be modified to violate FCC regs. Since 3rd party firmware allows such violations, devices get locked down.
How else are you going to explain the existence of separate US firmware which refuses to upgrade to international firmwares?
So why is TP-Link locking down their firmware even though they aren't required to? You have to ask them.
If you read TFA you'll learn that somebody did just that.
Adam Longwill 09:26:19 pm Wow. Thank you. I'm impressed with your digging
And this limitation is due to FCC rule clarifications in 2015?
And not some other reason
Camille 09:28:15 pm Right, due to FCC