Creating a site where buyers can browse property listings does harm the Craigslist business model. To say otherwise expresses a lack of understanding of online ecosystems.
As far as I can tell, they aren't posting the full listing. They are only aggregating the material facts about the listing. As far as I can tell, short of actually hoofing it to the place, you still have to get on Craigslist to make contact with the landlord.
What Craigslist is afraid of (probably) is Padmapper becoming the first place most people turn to when looking for an apartment. Once that happens, Padmapper can cut Craigslist out of the loop by inviting landlords to make Padmapper listings instead of Craigslist listings. (Note that once Padmapper become to "go to" place for apartment hunters, Padmapper's refusing to continue to host or "index" Craigslist listings helps towards that goal.)
Regardless the morality or legality of Padmapper's latest action, it is pretty clear to those of us who have spent a lot of time observing online ecosystems and software ecosystems that Padmapper's latest action is not in the interests of Craigslist since (even if Craigslist is not out to make money, but rather out to influence the world for the better) the only important source of Craigslist's potential for making money and of Craigslist's influence is Craigslist's being the first place people turn to when apartment hunting and doing other important things.
That is why Craigslist is against Padmapper's latest action even if Padmapper never posts the full Craigslist listing.
