I just stopped running in the opposite direction after seeing this photo long enough to comment on the photo. The concept is good, educate the consumer is a very important way to keep your customer base and have a credible reputation. An important evaluation for a retailer is what will be the return on investment by giving up selling space for demonstration space. I do not believe that this item can generate a great amount of volume and illustrating another building material such as deck stain sample colors would bring more dollars into the register.
Retailers have a moral and legal obligation not to mislead a consumer. This footing does not meet any code I am familiar with. You are correct in saying this needs to be a minimum of 3′-6″ below the ground to prevent it from breaking up and cause a failure in the deck structure. For the record, I have found more home closings turn to disaster because the deck was not filed and did not have an inspection to declare it safe.
Back to retail, love the concept, pick your battles, and be sure the message will not lead you to a litigation that could put you out of business.