Enterprise Rental Car won't charge you for not charging...I don't believe it.
They forget to tell you that the Volt, like the Prius, is a high price rental. At $100-$200* per day, I would still expect to be charged if I don't. Now to find an extension cord to go from the hotel room to my car.
* Rate based on added options, insurance, and how well you bargained you daily rate.