It's interesting that retailers would appear to allow this. If I were the retailer, I would require an agreement on who was allowed to run ads in my store, and

It's interesting that retailers would appear to allow this. If I were the retailer, I would require an agreement on who was allowed to run ads in my store, and get a cut of it. You're already on a very thin margin as a supermarket retailer. Why risk a "bad" advertisement, and then not get paid for it?