Can you provide a source for this assertion? The terms of service on their website has the standard "you automatically grant us a license so we can distribute it" clause, but then has this additional term directly following:
> You also expressly grant and assign to CL all rights and causes of action to prohibit and enforce against any unauthorized copying, performance, display, distribution, use or exploitation of, or creation of derivative works from, any content that you post (including but not limited to any unauthorized downloading, extraction, harvesting, collection or aggregation of content that you post).
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.
Ownership of the content remains with the person who posted the listing on Craigslist. Craigslist has no recourse via copyright law.