Under the common law doctrine of strict product liability as adopted in most U.S. states, a seller can be held liable if its product is unreasonably dangerous when put to a reasonably foreseeable use, a user puts the product to a reasonably foreseeable use, and the user is harmed as a direct and proximate result of the unreasonable danger. It's call "strict" because liability can be established without proof of negligence. The doctrine also affords a remedy if the seller has failed to provide adequate warnings and a user is harmed as a result.
It is conceivable, although highly unlikely, that 20 or 30 years from now long-term inhalation of PG in combination with certain other e-liquid ingredients might be shown to be significantly harmful. This could result in class action litigation similar to asbestosis and mesothelioma litigation which has bankrupted scores of companies and has cost tens of billions of dollars. Dow has gross sales of over $57 billion per year. Dow Chemical doesn't need to sell PG to e-liquid purveyors. They may have made a business decision that the market isn't sufficiently lucrative to justify any risk, however slight, especially considering the millions of dollars it would cost to defend major class action litigation, even if they were to ultimately prevail. Also, they probably do not wish to invite criticism and disapproval from the likes of Jay Rockefeller and Dianne Feinstein.
