Lets look at other media and see if your conclusion is correct. Do TV networks vet, produce and handle the ad-client and ad-publisher relationship, or do they mainly interact through marketing firms? The answer: they work mainly through marketing firms. The TV network is still ultimately responsible for the content they broadcast, as dictated by law.<p>Do newspaper work with individual advertisers, or do they work mostly through local marketing firms? The answer is again the same as above. Buying a news ad is commonly done through a marketing firm and the news paper is always responsible for what is printed.<p>Very few publisher in any media deals with individual advertise clients, and yet it works. Responsibility is done through contracts, through professional liability and standards, and as last resort through business insurance. As a result, its quite uncommon to see illegal ads on physical newspapers, on TV, on busses, and on other physical objects.