The real reason for the US to want records from Google?

According to James Dunnigan's StrategyPage.com (here), in 2005 the US was developing a system to search large masses of information according to concept, not merely by word.

The new concept search engine (CSE) was developed for counter-terrorism work. The CSE looks for connections between two people that it would normally take a human investigator to figure out. Given the huge mass of data on the Internet (including billions of words of material captured from instant messaging, chat rooms and emails), there’s no way humans could comb through it all for anything other than key words and phrases. The CSE makes sense of material, stores those interpretations and constantly compares all that it has found for connections between two or more authors, or members of the same organization (or way of thinking). CSE is expected to be a powerful tool in searching for terrorist activity on the Internet.

A month's worth of Google's search records would be an ideal test for something like this.