Craigslist's past method was to claim TOU violation on the part of PadMapper, denying PM ready use of its content.
Its goal is to maintain market position as the dominant classifieds listing service. TOU is just one way for CL to enforce its market position, and as a commercial entity, it's very likely it will continue to do so.
There's very little reason CL couldn't create an alternate legal claim based on content (the cartographic watermarking example comes to mind) to make multiple infringement claims against PM, which would invoke a whole host of copyright remedies ranging from injunctions and monetary damages to DMCA takedown notices (which could comprise the entire service depending on how construed by PM's hosting provider(s)). There's also the notion of a compilation copyright, in which a collection of CL postings could have standing. While Feist v. Rural Electric holds that a simple compilation of facts doesn't meet the minimal authorship requirements of copyright, a compilation of postings with some level of curation to CL might. I'm not aware of relevant caselaw here.
My own suggestion would be that PM pursue a dual strategy of coming to a business agreement with CL (PM's interface is vastly superior) while curating its own set of rental listings independently (CL is ripe for disruption here).
Disclaimer: I'm a long-time CL user and fan of much of its philosophy. I'm also a recent fan of PM. I hope the two will either work something out cooperatively, or manage to improve apartment and classifieds search by competitive means. I don't particularly care which.