Start With the Known Dangers 

We know how bad cigarettes are, so apparently it's fine to keep selling those.

San Francisco has officially banned the sale of e-cigarettes, even going so far as to outlaw online retailers from delivering to city addresses. In part, this ban was driven by a need for “more scientific investigation” into the effects of e-cigs. That sounds fine, except for what the city has not done: Banned the sale of standard cigarettes.