left / right on a SSBC NON slotted / drilled rotor?! Help!

The one and only reason I could imagine is if for some reason the studs are threaded left hand on one side and right hand on the other.
Hard to imagine on new aftermarket rotors, but I don't see one indication of why it would matter on the rotors you linked.

Maybe if you cross them the car turns right when the steering is turned left.:lol: