I'm reaching here, but maybe.... the police?
It's their jobs to track down criminals (be they online or offline) and prosecute them.
When it comes to copyright offenses, however, these are civil offenses, not criminal ones. (At least until the MPAA/RIAA push through legislation making them criminal cases.) This means that the copyright holder has to file suit themselves against the copyright violator. Sometimes this takes the form of a John Doe lawsuit until the violator's identity can be determined (or until the copyright holder has proven to a judge that they have enough of a case to pierce the accused's privacy).
The problem is that the RIAA is lazy. They don't want to file their lawsuits separately in the correct jurisdiction. They just want to file a huge batch of them all at once (as if they were related). They don't want to *actually* sue the person, just strong-arm them into a highly one-sided settlement. And they don't want to keep an eye out for copyright violators themselves, they want the ISPs to do their job for them.--
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