“Perhaps you’re correct, and I overestimate people’s intelligence, diligence, or both.”
I think it is very straight forward. A kosher symbol appears on a package, most people will assume the package has the hechsher of the kosher symbol. Unless you are interested in the ingredients, for allergy or health reasons, many people do not pay attention to these panels and lists when making their purchases. They scan the package, find the symbol and buy and eat. OU has an achryus to consumers who rely on their hechsher.