Look im a uneducated guy so I might use the wrong terms. I suspect these work like usb cables and you negotiate a rate of charge between the charger and car (that requires some energy but not enough to hurt anyone) and then the device internally flips a switch to enable charging.

If it was just live all the time I don't see how you bill anyone, they could just plug in, that also seems unsafe if say the cable is damaged or something.