Jumping on an unlocked computer is more of an "equal opportunity" thing. Only a handful of people in a company (IT) have the ability to remotely manage employees workstations. That creates an imbalance of power that needs to be respected. Using it for pranks against people who don't have that power isn't really fair.

That said, I don't really find the whole "recursihoffing unlocked laptops" thing all that funny either, but it doesn't bother me as much.