What is so dangerous about a can of coke that requires it to be taxed by the government?
Nothing really. Not if it is just one can. The problem lies not in soft drinks per se but in the lifestyle and over-consumption of it. Obviously the government cannot really tax "over-consumption" of soft drinks (there are too many vendors and ways to bypass the rules by simply getting your healthier friends to buy as part of their quota), not in our present society at least.
Hence, the second-best measure is to put a "fat tax" or a blanket tax on all soft drinks (or just on the upsized ones), so that people will be discouraged from drinking soft drinks. In America, the "fat tax" is specifically targeted at lower income groups who tend to over-drink these high calories drinks as part of their unhealthy but cheap diet.
As suggested, the fact that these individuals drink high-caloric soft drinks suggests that demand is pretty inelastic (There aren't many cheap alternatives out there and, hey, soft drinks are really quite nice to drink...). This means that the tax will fall mostly on the consumers, who aren't that wealthy to start of with. Analysts have suggested, and I tend to agree, that such a tax may not be that effective in reducing the quantity consumed by poorer folks in the short-run due to the income-substitution effect dominating the price-substitution effect (Giffen good effect). Essentially, not-so wealthy people may perceive that their income is even lower (it is in terms of purchasing power of soft drinks) and hence have a higher tendency to consume inferior goods like soft drinks. This effect counteracts the price-substitution effect caused by the higher prices of soft drinks. In the end, demand may still fall, but not by much.
In the long-run, however, it is plausible that beverage manufacturers or new entrants into the market will start to develop non-soft drinks that target the needs and demands of these social-economic classes of society. It is important therefore for the government to identify and clearly specific the content materials of soft drinks targeted by the tax. For example, a tax on corn syrup (which is reported to be the cause of unhealthy sugar content in soft drinks) will spur manufacturers to use healthier alternatives such as cane sugar.
An interesting point about corn syrup made by Willson was that corn is heavily subsidised as a crop in the United States. The heavy use of corn syrup (a cheap but unhealthy source of sugar) in the United States suggests that the real cause of the sugar problems may be the government subsidies on corn farming in the first place.