I got the impression that Google reading your documents was the source of distress in the parent comment.
Public documents in Google Docs are used as targets for spam or phishing campaigns pretty frequently. (The latter is usually by way of Google Forms, which can be used to drop content into a spreadsheet.) Google needs some way to allow this behavior to be flagged, and ideally to recognize it before it gets abused.
Private documents are another matter. Even there, though, I imagine they are obliged to scan some content, e.g. checking images against child pornography databases.
Nevermind my original comment then. That makes sense.
I'm sure it's just fine most of the time but I don't understand how people trust it for mission critical (or even just important) stuff.