Incidentally, I was reading an article about Activia on Slate, which purports to improve digestive function (e.g. help with constipation) with special cultures etc. It appeals to women because it supposedly helps with weight lost. Although, it seems these claims are not based on any actual formal scientific studies. So it goes.
I assume originally, yogurt was just made with whole milk. At some point, since fats are "bad", a switch was made to skim or low-fat milks as a base, and to compensate for lack of fat, corn starch and gelatin were introduced. And since yogurt (whole or not) tends to be pretty sour, sugars — or in cheaper brands, corn syrup — was added.
I did a little on-line research, and with single-serving brands like Yoplait caloricly, there's a lot of sugar calories. Yoplait, your typical supermarket yogurt, has 170kcal per serving, 15kcal from fat. Nancy's all-natural whole milk yogurt (with a little honey) has 170kcal per serving, 70kcal from fat. "DANNON® ACTIVIA" has 220kcal per serving, though it too is made from "reduced fat" milk; Light has 150kcal and it's made from non-fat milk. Doing a little calorie math, Dannon Activia Light must have about the equivant of 4 teaspons of refined sugar added to the milk.
What has happened is everyday yogurt has morphed from a high-fat, creamy substance into milk-flavored Jello.