Right. I've already agreed that there is a significant difference between displaying copies of CL listings and just having pointers to them.
It doesn't seem, however, that the link is the crux of CL's complaint. Rather it's the fact that PM has copied CL's data in order to generate a mapping of physical locations to CL listings.
You can't copyright data, only its expression. That a certain apartment is available for rent is a fact. The text that appears on the CL website is one expression of that fact. The map that appears on Padmapper is another expression of the same fact.
It doesn't seem, however, that the link is the crux of CL's complaint. Rather it's the fact that PM has copied CL's data in order to generate a mapping of physical locations to CL listings.