Google, IIRC, does not let you search from TOR either. If they can't track you, you're no good to them.
In my past experiences, if you're on tor (or proxy'd through a known portal), and have Javascript disabled entirely, Google will sometimes (not always) give you an error stating that your IP is a known abuser. If you DO have JS enabled, I have encountered the "Your IP is known to be abusive, so type in this captcha in order to continue" and then it forwards to encrypted.google.com.
I get similiar behaviour with VPNs & Google.
So you are told that the IP you are using is rate limited.