I agree the pragmatic approach is to research which devices are unlockable, as well as unlock them when you're still well within the return window.
But the general problem isn't whether it is possible for someone to buy an unlocked phone, but rather that it is possible to "buy" a phone and have it turn out to be non-unlockable. The general case is important so that users who run up against manufacturer shenanigans can straightforwardly route around them, and also simply preventing the pileup of more unusable planned obsolescence ewaste.
But the general problem isn't whether it is possible for someone to buy an unlocked phone, but rather that it is possible to "buy" a phone and have it turn out to be non-unlockable. The general case is important so that users who run up against manufacturer shenanigans can straightforwardly route around them, and also simply preventing the pileup of more unusable planned obsolescence ewaste.