Check out this story from Slate about why people get so frustrated about soda taxes. An excerpt:
Most of us would probably agree that we'd be better off if we drank less soda, but any government action designed to control consumption puts us on alert. A tax is far less likely to induce reactance than an outright ban, but we feel queasy whenever we think The Man is trying to influence our behavior. In the case of the sugar-sweetened-beverage tax, which has often been referred to as a "fat tax," our discomfort is greater because the measure seems to encourage finger-pointing.