Righthaven, as you pointed out turns on this very issue: Righthaven was not able to enforce someone else's copyright because Righthaven did not have a license to use the copyrighted material. The Righthaven court found that the "license" Righthaven had was merely a "license" to sue for damages, but Righthaven was not actually allowed to use any of the newspapers content.
Craiglist is different. Craiglist has the right to use the copyrighted content of its posters. It is a basic tenant of copyright and contract law that Craiglist therefore has the right to enforce the contract to protect its own license rights.
Craiglist is different. Craiglist has the right to use the copyrighted content of its posters. It is a basic tenant of copyright and contract law that Craiglist therefore has the right to enforce the contract to protect its own license rights.