The issue is a few have exploded and if someone sees puffs of smoke rising from a few rows ahead, they may assume it is ok to smoke the real thing and light up.
I'm not one fir getting involved when I'm not the operating crew but I did see a pax in the window seat using one and trying to hide it. I had to politely tell her to stop. She claimed she didn't know they weren't permitted (then why were you hiding it?!).
I think they could set off an alarm. Right now I'm in a hotel room with a big sign saying to close the bathroom door when showering so as not to set off the alarm. It's not unreasonable to assume that someone standing in a tiny toilet in a plane could exhale the vapour into the sensor and activating it. I know for sure that deodorant can do this.