It sort of seems like a critique of the terminology if dark arts tend not to be deployed by the dark side.

I agree. I think the dark side terminology is based on the "dark side of the force" from Star Wars, which has connotations of a personal fall into temptation, and the dark arts refers to magic of evil intent or effect, perhaps from Harry Potter, where it is used by evil but not self-deceiving villains. This could explain the inconsistency.