If a person is attempting to maximize their health, there are behaviors that have a better cost:effectiveness ratio than buying organic.

Um, are you considering convenience and other akrasic factors under "cost", and allowing for perceived or anticipated "effectiveness"? If not, your analysis is probably incorrect. (Heck, it's probably just incorrect when you consider what information people have about these behaviors.)

(People could just be systematically acting in a sub-optimal fashion, for instance) but given how much of what people do is signaling behavior, it is my baseline explanation.

You should probably update more of it towards the parenthesized hypothesis, i.e. people behaving in sub-optimal ways by default. If this weren't the default, there'd be little need for Less Wrong.