[uiri]

Many ISPs abuse DNS to serve ads. One method is to rewrite all NXDOMAIN responses to an ad server. Another involves injecting ads into HTTP responses by hijacking DNS.

Although your points about capability and motive are valid, I don't think that Google does feed DNS queries into a database. Given all the evil DNS servers out there, I think that it is in Google's best interest to provide a clean alternative and contribute to better internet infrastructure.

[dingaling]

"After keeping this data for two weeks, we randomly sample a small subset for permanent storage."

https://developers.google.com/speed/public-dns/privacy