Probably because of the sensors used to track charges for anything that's removed (or in some cases, moved). It's not like they can trust the staff to keep count.
Or maybe the minibar fridge was plugged into that outlet? The hotel does not want the next guest having warm drinks in his minibar. It could take ages for the thing to cool down again.
I could see the next customer getting really PO’d if the minibar fridge had warm drinks in it.