Enforcement doesn't have to be perfect to benefit society. Once it was illegal to advertise drugs to the public. After it was legalized suddenly drug company budgets were overtaken by marketing, instead of R&D. In my opinion that was a net loss for society.
From what I can tell 70% of drug marketing budget is about persuading doctors and not consumers so I don't see how your example applies. In fact it just shows that companies will find a way to spend marketing money to influence people given enough ROI even if they can't target consumers directly.