A shift in responsibility
"parental lock is entirely uncontroversial, as it is not government but parents who decide"
That certainly *shouldn't* be uncontroversial.
In the absence of locking, broadcasters know that anything they might transmit may be seen by impressionable folks, and they also know that courts will take that into account if matters go that way. With locking universally available as a standard feature, it becomes possible for broadcasters to argue that it is the parents' job to configure tellys properly and so the broadcasters can transmit anything they like as long as it is properly tagged.
That may or may not be a Good Thing. It is certainly a significant change to the regulatory environment that has prevailed in just about every country since the introduction of TV.