George Orwell may have nailed the issue with George Osborne's new sugar tax with a book published in 1937.
The Road to Wigan Pier is a collection of Orwell's observations and essays about the working and middle classes during the Great Depression, chapter six of which discusses the food eaten by the mining communities.
It describes how they have enough money to buy food, just, but usually buy with the intention of bringing excitement to their lives rather than for nutritional purposes.
Would it not be better if they spent more money on wholesome things like oranges and wholemeal bread or if they even, like the writer of the letter to the New Statesman, saved on fuel and ate their carrots raw? Yes, it would, but the point is that no ordinary human being is ever going to do such a thing.
The ordinary human being would sooner starve than live on brown bread and raw carrots. And the peculiar evil is this, that the less money you have, the less inclined you feel to spend it on wholesome food. A millionaire may enjoy breakfasting off orange juice and Ryvita biscuits; an unemployed man doesn’t.
Here the tendency of which I spoke at the end of the last chapter comes into play. When you are unemployed, which is to say when you are underfed, harassed, bored, and miserable, you don’t want to eat dull wholesome food. You want something a little bit ‘tasty’. There is always some cheaply pleasant thing to tempt you.
However, Orwell goes on to discuss, in condemnatory terms how, in his view, the practice led to malnutrition and physically impaired children - so it's not a ringing endorsement either.
The point remains - the public aren't buying fizzy drinks because they think they are the best health choice, and that premise has been consistent for nearly 80 years, according to Orwell, at least.