Submission + - Who owns your files on Google Drive? (cnet.com) 1
"Your Content in our Services: When you upload or otherwise submit content to our Services, you give Google (and those we work with) a worldwide licence to use, host, store, reproduce, modify, create derivative works (such as those resulting from translations, adaptations or other changes that we make so that your content works better with our Services), communicate, publish, publicly perform, publicly display and distribute such content.
The rights that you grant in this licence are for the limited purpose of operating, promoting and improving our Services, and to develop new ones. This licence continues even if you stop using our Services (for example, for a business listing that you have added to Google Maps)."
The last sentence makes all the difference. While these rights are limited to essentially making Google Drive better and to develop new services run by Google, the scope is not defined and could extend far further than one would expect. Simply put: there's no definitive boundary that keeps Google from using what it likes from what you upload to its service.