> I doubt this as well, since the "with vitamins" badges are also on food that's way beyond necessary spending, from gum to expensive cereal, all the way to power drinks that cost as much as a Starbucks latte.

Well, I guess that's just basic marketing? Vitamins are approximately free to add, and some people are more likely to buy stuff that makes vitamin health claims, while approximately no one dislikes vitamins enough to stop buying a product because of vitamins in it.