My understanding is that one of the differences is that 'additive-free' tobacco is at least free of additives designed to modify the availability or metabolism of nicotine.
In particular, if memory serves, the use of ammonia was a big deal during the tobacco litigation because it demonstrated manipulation with an eye to 'drug delivery' (and, for some reason, 'big tobacco' was trying to claim they didn't do that). Something about it managed to get the nicotine into the bloodstream faster, damn the other consequences (more nitrogen compounds?), and you can really taste it in some brands -- like Dad's old Trues way back when, whee.
So it's not like a 'natural' cigarette is non-addictive, but at least they haven't kicked it up a notch. Compared to 'tweaked' cigarettes, the nicotine kicks in a little more gradually, and 'vaping' is supposed to have a slower delivery than smoking, so maybe it's easier to slide over from non-ammoniated than from Newports or something.