I started to notice that Google has included search results pages of many high power websites like Ebay and Amazon.
As far as we are told this is still against their own guidelines!
Use robots.txt to prevent crawling of search results pages or other auto-generated pages that don't add much value for users coming from search engines.
Google is showing my own sites internal search pages in the SERPS as well but I've had them blocked for years.
With big sites like Amazon and Ebay its easier just to search vs. digging though their navigation. I wonder if something changed where they feel some sites search terms do indeed return valuable results and allows it to happen.