Out of curiosity, how can they enforce that sort of thing? Blocking ip's is the only thing that comes to the top of my head, but some tricky javascript might be able to keep google from even knowing that they are being crawled at all. How could google even find the people doing it? (Besides, you know, reading this article.)
If you start doing automated requests to their search engine, very soon they start serving captchas. They detect it using machine learning on features like number of requests coming from an IP, etc.
