IMO it perhaps isn’t the more elegant of solutions, but it’s a decent/easy enough one.
We have a national cash-flow problem, more tax revenue will help it, well, it won’t hurt anyway.
We have an obesity problem – if presented with two bottles of pop, the one with 15g of sugar in it for £1.50 or the one with no sugar in it for £1 hopefully, some at least will take the sugar free one.
It’s simple to understand and simple to impose – the one downside I can see is that there’s a huge mark-up on fizzy drinks, it’s possible the makers could reduce the cost of the sugar versions of their products (citing lower production costs etc) to achieve parity in their pricing – to what end, other than ‘evil’ I don’t know, but they could.