What is an ad? Can the beer cans say "Coors"? Can airplanes fly by with ads trailing? Can people's clothing display brands? Can the players' shoes have the Nike swoosh? Can pop-up shops sell their goods at the store or give out samples? Can a company buy tickets just to plant attendees who will loudly wear their brands? These are all forms of marketing. If they are permissible, then how do you enforce it? No billboards over a certain size? No audio or video ads?