Also, as others mention, they can and do monkey with the results.

In other words, here, your ISP is in part a hostile entity. At least, in my perception -- and I'm not alone.

P.S. Of course, there's the argument against giving Google all your DNS usage, as well... I use a different DNS service from a company with a good reputation that says it's not collecting usage data. Even then, and with the state of our State, who knows...