PadMapper gets content from more than just Craigslist. They have their own postings (padlister.com), sublet.com, apartments.com, apartmentfinder.com and a lot more. There's really not that much difference between them and a search engine.
Several innovative solutions have been invented over the years, the most obvious one is to do the CL scraping on the client, this way there is no indexing server to block, it's just the client reading all the search listings, parsing them and offering faceted filters. It breaks the spirit of the TOS but it would be impossible for CL to block. I am not advocating this, just pointing out one of the many approaches that have been attempted over the years.