I've come to the conclusion, based on extensive speculation, that the FDA is simply trying to ensure that e-cigs don't get classified as 'smoking cessation aids' but are classified the same as cigarettes, so they can be taxed hard, because as cessation aids they cant justify ridiculous taxing, and that that is really all this is about. The real problem is that e-cigs are just too good for smokers. If the FDA could prove somehow that they caused just as much cancer and death and misery as analogs, then they'd probably get passed and approved (with appropriate warning labels (and taxation)) with ease.
Edit: when I'm talking about taxing the e-cigs I'm talking about the same sort of taxing they have on cigarettes, not some reasonable low level tax.