Your situation in the US may be different than my country's, but in our case, our cities are either running deficits or are cutting close. The federal and provincial governments have cut taxes, meaning that cities are not getting the financial transfer payments they need; at the same time, those same governments are downloading responsibilities onto cities which is furthering the problem.
Naturally, a city needs to find a way to make up for that income, and taxing things like soda is the best option within their limited jurisdiction.
You guys will need to get used to this sort of thing, more and more. Especially if your country establishes a public health system. The health of the nation will become everyone's (taxpayer's) business, and unhealthy habits and lifestyles will have an impact come tax time. So why not create incentives that deter unhealthy living?