I don't know the legal degree of liability for adverts, either in print or on the web. However if what black hats do is illegal* then the law should apply to everyone, and that could just be a good thing.
Visitors to PoF get malware. They sue PoF. PoF in turn sues the add slinger. The add slinger then has plenty of incentive to make sure the next bunch of adds they sling aren't malware vectors.
* Personally I think the internet should be the wild west with no laws governing what you can and can't do but I do acknowledge that's not the world we live in.