I've seen plenty of places where there are two charging spots per charging cable. In which case it makes sense to unplug a car that's in the spot and full.
It sounds like it makes sense until someone else wants to charge their car regardless of whether yours is charged or not. That will be 99% of the use case for this. Just charge idling fees, that's a great deterrent for sitting there with your car fully charged and still plugged in. I'm not sure about all of the major networks, but Tesla and EA both charge those fees.
