I think people are reading too much into the significance of the flowers. The way I saw it was, flowers are a generic decoration that you wouldn't associate with the coffee, whereas the eyes make people think "What's with the eyes? Big brother or what? Oh yeah, the coffee..."

Be interesting to repeat this with some method of working out whether more people were paying up, or whether the people who pay were paying more. Maybe a secret camera, but that might defeat the object by validating people's paranoia.