I think soda is probably one thing that contributes to obesity, and we all know that obesity contributes to health problems, and easily preventable health problems end up costing society money...so I guess I agree, in principle, that if society's gonna eventually pony up for a case-a-day soda drinker to get a quadruple bypass at 50, then yeah, he should be chipping in a little extra on it along the way. It's very similar to levying extra tax on smokes when you consider what smoking costs society..
However...taxing non-diet soda alone seems a bit arbitrary. What about drive-thru food, in general? Or bacon? Lard? Diet soda, even, since it's also pretty horrible for the human body.. If you're gonna tax one bad thing, you should probably tax them all.. The problem is, how do you decide what's good and what's bad, considering "leading scientists" change their minds on what's good and what's bad from one day to the next..
All in all, I just don't think it's feasible.. Good intention, poor execution.
On the upside...maybe if they decided to arbitrarily hike the tax on regular soda, manufacturers would work extra hard to create a diet soda that didn't taste like butt on a dry cracker.