An externality is a cost someone incurs that they didn't consciously choose. They can absolutely be involved in the trade. For one thing, excessive sugar intake is a blight on children. Childhood obesity is an epidemic on this country fueled by the corn industry. Excessive HFCS is in everything from applesauce to bread to juice boxes. It's hard to argue children are actively involved in the trade.

But it applies to adults too who purchase the goods. Most people if you ask them don't realize how much sugar is in the food they consume, and they don't understand the connection between sugar consumption and heart disease. No one chooses heart disease.

People can read a percentage or a gram count on the packaging, sure, but the nature of sugar is that of addiction. It's not just that people like sugar, it's that we are wired to want to consume it, and Big Sugar takes advantage of that by loading a surprising amount of our food with too much of it. Many people are surprised to learn their bread has sugar in it. Even with soda they are surprised just how much is in a can if you measure it out physically on the table in front of them. People aren't well-informed about what they are consuming and the long term health related effects, and even if they are they are too addicted to stop. Big Sugar knows this and takes full advantage, because millions of people dying every year does not affect their bottom line (and they can easily replace their old, dead customers with new, young ones)