[gwd]

It's a market-based solution to a market-based problem.

A classic problem which make markets inefficient is "externalities" -- the cost is artificially lowered because some of that cost is "externalized" away from the people selling the product. This artificially low cost causes the markets to allocate resources in a globally non-optimal manner.

One solution is flat regulation -- you can't have more than a certain amount of sugar in your drink. But that's unreasonable and unpopular.

Taxing is a market-based solution: it forces some of the external cost to be borne by the person selling it. This allows the market to allocate resources based on the actual cost, rather than the artificially low cost, while still maintaining the flexibility and diversity of the market, rather than having to impose regulation.