They are plenty of cases where copyright enforcement is voluntarily handed over to a third-party: for example, Hollywood to the MPAA, or the BusyBox developers to the SFLC. Does anyone know what the difference is?
Righthaven is different. Righthaven did not have a copyright use license, so it did not have the right to enforce the copyright. (A copyright enforcement "license" is a non-license, legally, since it does not convey any actual copy "right" such as distribution.)
Craiglist does have a copyright use license, and thus has the right to enforce the license it has been granted.