I've heard this argument many times before, it was the same with Oodle, but PadMapper's business model is to aggregate apartment listings (Google's is not) and this is in contradiction to the TOS so inevitably Craig Newmark sends them a C&D letter.
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.
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.