The issue is that if the tax is successful, and revenues start to fall from that particular source due to fewer sugary foods, there may be an instinct on the pa

The issue is that if the tax is successful, and revenues start to fall from that particular source due to fewer sugary foods, there may be an instinct on the part of politicians to make up that money elsewhere. A good example in the US is cigarette taxes - while originally designed to fund cancer treatment and anti-smoking initiatives, a lot of states have come to depend on the revenue from those taxes. As more people switch to e-cigs, which are not taxed and offer big cost savings, states are talking about taxing e-cigarettes similarly. If the goal of the cigarette tax was harm reduction, presumably there's some incentive to have people smoke e-cigarettes instead of regular tobacco - but the real concern is the loss of the tax revenue the state has come to depend on. Ringfencing (theoretically) discourages this.