That's a pretty farfetched argument, since there's one big difference between Google and the scraper sites: Google is doing the heavy lifting of being a search engine, while the scrapers are just lifting the SEs' results. (Copyright issues come into play here, too: Google's capturing of page titles, text snippets, etc. is almost certainly protected by the "Fair Use" doctrine, which allows excerpting for review purposes. It's unlikely that a court would extend the same kind of "Fair Use" protection to a scraper site.)
As for why Google tolerates scraper sites, the obvious reason is a need for ad inventory. It wouldn't surprise me to see Google trying to shove the scrapers down in its search results (after all, search is Google's core product and source of "eyeballs") while allowing scraper sites to remain part of a cheap run-of-network base product (the existing AdSense 1.0) as higher-priced AdSense variants are added to the network for more demanding advertisers.