How on Earth is that a double standard? That's just how HP work and have always worked.
The double standard is in treating "damage despite a successful save" as categorically different from "damage despite missing your attack roll".
Strong disagree.
Look, even the weakened form in 5e, saving throws represent active resistance. Something happened (poison, fireball'd, etc.) and you can "save" for no effect or partial effect.
On the other hand, attack rolls are different. They are traditionally binary; you either succeed, or you don't. If you don't succeed on an attack roll, the effect doesn't happen.
Here's the way to look at it-
Monster Ability- Roll to hit. If hit, the target is poisoned for 3 rounds. If missed, the target is poisoned for 1 round.
Compare-
Monster Ability- On a successful hit, the target must save. Successful save means the poison lasts 1 round. Unsuccessful save is 3 rounds.
There is a MASSIVE conceptual difference between those two things.
What this really is about is that people think that "auto-hit" spells are cool, and want to give similar things to martials. Fine. Give the martial "Unerring Strike" X number of times per day and provide that the target takes Y damage, and 1/2 damage on a successful save. THAT is similar.
Damage on a miss isn't. Different mechanics. Bad analogy. Hit points and saving throws are not the same thing.