Lithium-Ion batteries have a history of overheating, some tragically with the loss of the aircraft and death to passengers and crew.
South African Combii over the Indian Ocean, UPS 747 in the UAE, Korean 747 of the coast of Korea. (Last two were pure freighters, which is why you probably haven't heard of them.)
Li-I batteries are in a plethora of devices carried onboard aircraft, generally (fairly) expensive devices made to high quality standards. Even then there are continued reports of overheating and fires, rarely in flight, thank goodness.
E-cigs are cheap, some made in factories with no concern for quality control and, as we have seen here, fairly easy to modify. They can be very dangerous. Much more so than a phone or a laptop.
I would prefer that they were a forbidden device in the cabin and the hold, but realistically that isn't going to happen. But to use them inflight is too big a risk. Onboard fires can and do kill everyone on board. Do we really want to take the risk just to satisfy someone's nicotine addiction?
Many aircraft have power to seats, and I expect that having them at every seat will be the next big thing in aviation. So we are going to see them charged on board. Scarey