I don't think that's the loophole, actually. Since technically as far as Sony cares you license everything, the physical disc just gives you a "license" to use the software on it.
No, the distinction is that the "unlock" is "DLC" and the full version is a "game."
One of the "perks" of a PSPlus subscription (in fact, the only perk prior to the PS4) was that you'd get discounts on the PlayStation Store and that sometimes you could get free games. If you got a "free" game through PSPlus, you only could use it while you have an active PSPlus subscription.
Well, guess what? The demo version of this game is, in fact, a "free" PSPlus game. It's only available to PSPlus subscribers. So as soon as you drop your PSPlus subscription, you lose access to the demo.
And the "full unlock" is DLC to said "free game," so if you drop your PSPlus subscription, you lose access to the entire game as you can no longer play the "base" game (the "demo").
This whole thing strikes me more as laziness than out-right maliciousness. Someone realized it screwed people over, and then rather than try and fix it by making an exception, Sony said "fuck it" and reverts to screwing customers over, in typical Sony fashion.
Still malicious (they could fix it, after all), but with a nice helping of being too lazy to fix a problem they clearly recognize exists.