The power points onboard are to power devices whilst being used, charging is prohibited as the build up of heat can cause fires, this doesn't tend to happen if the device is being used. As the e-cig cannot be used, the cabin crew was correct in what they said. Similarly if someone is found to be just charging a device such as iPod/laptop etc... This will also be challenged. If its plugged in, it must be being actively used.
All airlines I have used recently with USB or traditional sockets have this rule.