Of course Google reads and aggregates data inside your private docs. How would it provide search over your documents otherwise?
Some other engines don't do this, and the difference is remarkabe. Try a full-content search in Windows 7, you'll be staring at the dialog for two minutes while it tries to find a file that's in the same directory as you started the search in.
The poster clearly thought about search in terms of the existing Google search functionality which is near instantaneous.
Usability matters to the average end user and a delayed search is not usable for most people.