You'd get some of that effect, even if the drug was actually a placebo. But if it's working, then people have an even better motivation to make changes.
Yogurt is widely seen as healthy, so people who want to change to a healthier lifestyle might pick it.
(Yogurt might actually be healthy, I don't know. That's almost besides the point for the effect here.)
sweetened Yogurt with fruit or chocolate or caramel or whatever on the other hand is basically a dessert and not particularly healthy.