I was always wondering that. In essence, both cases appropriate CL's data for the users benefit, and both tend to send traffic back. I'm currently in a similar situation, where I have to ask myself how much i can scrape/aggregate/visualize 3rd party data that is useful to an user while stil providing value to the 3rd party.

I believe ultimately, it is at the 3rd partys (in this case Craigslist) discretion who is allowed to appropriate their data and who not.