https://www.fcc.gov/news-events/blog/2015/11/12/clearing-air... TP-Link may well be locking down firmware, but it is NOT because the FCC is forcing them to.
How do you know? 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? http://www.dd-wrt.com/phpBB2/viewtopic.php?p=990535
So why is TP-Link locking down their firmware even though they aren't required to? You have to ask them.