This effect makes me very curious, because yeah I wouldn't think adding sugar would be better, but one effect could be that - if people are really only using a teaspoon of sugar, maybe people who are able to moderate the sugar intake in coffee are good at moderating diet in other ways as well? I drink black coffee, but, it's not that I'm being healthful, it's I like the dark strong flavor of coffee, and that desire for "strong flavors" means I don't always make the best health choices in other areas.
