I think this might be a disconnect between what those lights are "really" for, and how they're actually used (de jure vs de facto, in a sense).

They aren't meant to have a specific meaning, they're just headlights, but when going in reverse. So if the car has a feature to "turn on the headights" it makes sense to activate the ones on the back too.

Though that's just pedantry that kicks the can down the road to the question, why are the headlights turning on with nobody in the car?