Craiglist doesn't need to write those posts. Under their TOS, Craiglist has a license to the posts (which are copyrighted by their owners ). This license gives Craiglist the authority to enforce copyright against persons who interfere with Craigslists' license in the posts. Thus, the last sentence of their TOS is perfectly legal and enforceable. The license would not give Craiglist a license to do anything to other services on which the original posters also post their listings to. But that is not the case here.
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.
