I have noticed that a rule that has been copied will show up under a device's "In Use By" pane when the rule currently has nothing to do with that device. In my case, this seems to be an artifact of the device that the rule was controlling before the device was changed to a new device (which uses the same rule logic).
This is a known consequence of cloning rules and has been mentioned in several posts.
At the moment I've not seen anything that fixes/stops this I'm afraid.
Yes, from what Bruce has said it isn't impacting the rule at all, it's just a db artifact. He's certainly aware of it, as enough of us with acute OCD over it have bugged him.
My guess is that there is still a condition that remains from the original rule that is no longer used and could be deleted that would then release that rule from the in use by?