I know and don't care that the ability can be countered with spells because my initial grievance is that a passive ability can be removed and ignored constantly because it counts as a status effect rather than an actual passive ability.
But they aren't counted as status effects. If an enemy casts dispel on a character then the passive resistances remain.
Th dragon's skill is just a special Boss Ability, which changes your resistance/etc into a weakness. The 1 slot elemental resistance abilities weren't designed to be that good and override it.
Or as a bad comparison: You can boost your Speed with passive abilities, but that doesn't prevent the enemy from casting Slow and managing to act first still. ---