Because a lot of spammers/SEO Scrapers (such as Scrapebox or xrumer, or even sites with private databases such as SEOmoz or SEMrush-like databases could) use TO

Because a lot of spammers/SEO Scrapers (such as Scrapebox or xrumer, or even sites with private databases such as SEOmoz or SEMrush-like databases could) use TOR to bulk-scrape Google. So you are told that the IP you are using is rate limited.