Google is suing "rogue pharmacies" who use their search engine ad system to scam searchers. Google explained:
It’s been an ongoing, escalating cat-and-mouse game—as we and others build new safeguards and guidelines, rogue online pharmacies always try new tactics to get around those protections and illegally sell drugs on the web. In recent years, we have noticed a marked increase in the number of rogue pharmacies, as well an increasing sophistication in their methods. This has meant that despite our best efforts—from extensive verification procedures, to automated keyword blocking, to changing our ads policies—a small percentage of pharma ads from these rogue companies is still appearing on Google.
So Google is suing them to make an example of some in order to "serious deterrent to anyone thinking about circumventing our [Google's] policies to advertise illegally on Google."
I think the point is that Google does not want to be seen as aiding and abetting illegal activity, and this makes it clear which side of the fence they're on. It's one thing to say "they're doing it even though we disallow it and try to stop them but still collect their money in some circumstances where we're ignorant of what's going on", it's another entirely to say "we disallow it, and when we find out about it we sue them".