You don't need to police every comment. You just need to randomly poke at things, and make an example of anyone who lets something slip thru. Then individual businesses are going to be scared into policing their specific turfs. And when it's decentralized like that, it's quite possible to police every single website on the Internet (or at least on your section of it).
Why do you think so? The question isn't whether sites distributing pirated stuff exist - the question is, how many more would have existed if not for policing, and how many people would have used them that are deterred under the existing regime.
Effective policing generally doesn't mean that people can just flout the law whenever they want. That would suggest to me that particular law cannot be effectively policed. However I'm willing to concede that "effective" is a subjective term so we could both argue our points are correct.
