There is info about this at
http://www.scroogle.org they are calling it "search scraping"
The site shows who the real google top 20 are in many catagories compared to who google actually shows as the top 20 after running their "scrape filter". The sites that are highly optimized for ecommerce keywords are the most filtered. The site keeps a live list of the keywords that are being "scraped" the worst. So in fact they are scraped because they have done the best job of optimizing their page for google.
The site owner is speculating that Google is doing this to cause those site owners to buy into Adsense in order to get front page results.