The problem is that every ad network wants some of that sweet sweet revenue stream but nobody wants to take the responsibility of securing their ad networks against malware and vetting them against some minimum standard of ethics.
So we get "promoted stories" like "38 Photos of Criminals that will Give You Nightmares!" and "WARNING! Your Android device has spyware! Install Mobogenie!", we block the ads, and the website owners are caught in the crossfire.