Surely it would be to Google's advantage to scan as much as they can in the background for targeting ads, even if they actual search is activated on demand.
So because Google is currently not taking the tiny step that would allow them to collect even more data (even though their business model is all about spying on people for all the data they can get), this somehow say anything about what their actions will be in the future? Just like how Google still has the "Don't be evil" slogan?
They promise[1] not to abuse the software that just happens to require hooking into the OS in such a way that they could spy on everything. Google's pattern has been to introduce products that people find useful, then log everything they can get access too once they have a large enough market share (usually taking advantage of the general ignorance about what can be done with the modern networked computer).
To suggest that somehow this time Google will act differently is to ignore the last decade of history and their publicly stated business plan.
[1] ...actually, I haven't seen such a promise, just people assuming so. Maybe I missed something.