A goblin is behind a rock that gives it half cover. It shoots its crossbow at an adventurer in the open, and the adventurer returns fire with a bow. For the sake of argument, the goblin, the rock, and the adventurer are all in a line on the grid.
Alternatively, the goblin is in a guard tower with a low wall that gives half cover, with the same scenario.
In both cases, who has cover from who?
Obviously, the goblin gets +2 AC against the adventurer's attack, because it's in cover. But does the adventurer have +2 AC against the goblin's attack, because there's half-cover between them?
One argument for the bidirectionality of cover is that it works with full cover; if the rock was big enough, neither combatant would see the other, and thus have full cover from each other. However, it seems weird and unintuitive that attacking from a protected position would give the attacker a penalty.
If the examples I gave don't result in an advantage for the goblin, is there a scenario that does?