ChargePoint chargers think that you've unplugged once charging is done, at least on the Model S.
Also, note on the Chargepoint.net site, if you click on Connections, Verdek EV Solution is offering free EV charging in exchange for access to your charging history. There are other offers/discounts available on that page. Regardless, nearly all the ChargePoint stations around New England are free (at least, for now).