I'm not getting into the debate of pro vs. con on the information contained in the article below, just posting it as I saw it a couple of days ago and it seemed relevant to your post. I think if it's an unknown, Disney makes the right choice to take a conservative approach.
Quote:
Secondary Exposure to E-cigarette Emission Might be Harmful to Health
Electronic cigarettes are not emission-free. E-cigarettes contain volatile organic substances, including propylene glycol, flavors and nicotine, and are emitted as mist or aerosol into indoor air. Study showed that these ultrafine liquid particles of less than 2.5 micrometer in diameter may penetrate deeply into the lungs. Study further showed that these e-cigarettes produce substantially fewer ultrafine particles than conventional cigarettes, however, the substances emitted by e-cigarettes may be inhaled by non-users when used indoors. Second-hand exposure to e-cigarette emission which may lead to adverse health effects cannot be excluded.