The copyright is still with the post author; Craigslist is not asking for it to be transferred. The author still has the ability to license the content to the other sites. In doing so, they breach the agreement with Craigslist saying their license was exclusive. So it's simple breach of contract between the listing author and CL; the other site has done no wrong and is not a party to the breach.

IOW: This text gives Craigslist some ammo to go after a site they can prove got the listing from copying Craigslist, but not sites the listing author also posted to.