> if every poster were labeled “AI generated” with a black banner at the bottom, people wouldn’t care.

Do you have evidence that people would care?

The specific case of this poster it appears they are claiming it was disclosed by the submitter that it was AI and still picked. Maybe that's diffusing the situation after, but it seems most likely these people selected and asked to care about the poster knew it was AI and still selected it. Most people would care less than they did.