Earlier quoted context omitted.
Nobody would buy something without an understanding of it being beneficial. I'll grant you that understandings can be faulty. One may learn that there was no benefit. But that's outside of the ad itself. Invalidating your hypotheseses is an important part of the scientific method.
Explain sugar cereals, soda, cigarettes, alcohol, drugs, gambling, etc. The advertisements create the misunderstanding. "Lucky Charms are part of a complete breakfast" etc. No normal person under normal circumstances would think that marshmallows or cookies are part of a healthy meal if you asked them directly to think about it.
Who is buying Lucky Charms because they find them disgusting, but somehow see them as being important? Let's be real: People only (continue to, at least – they may buy once just to try) buy Lucky Charms if they like consuming them.