Bad arbitrage is prohibited?

Question

contextual advertising

I can not understand. There are people littering the Internet with this magical weight loss or something, a lot of them. In fact, it is not regulated in any way. Gray zone, on the edge. It's like selling amulets or amulets. Such ads are blocked almost everywhere. But in itself, this activity on the Internet is very common, and it seems like in reality the responsibility as a seller of amulets. Legal business?