It's the competing incentives of a company that provides data services to end-user customers and a company that mines as much information about people to sell advertising to companies.
Without strong controls about what they can do, we are always at the whim of what they might do. Google feels like a fairly bipolar company from the outside, because they present two faces depending on who they are dealing with, end-users or companies looking to advertise.
As an end-user, Google knowing all the little details about everything I do and many places I go (because analytics JS, G+ button inclusion, etc) is disconcerting. For a company looking to advertise, them not doing this all of a sudden would be disconcerting, and they would probably look to some other company that is doing so. It isn't just Google. Facebook knows a startlingly large amount about you too.
I'm increasingly convinced this is one of those places where the market is failing us because the negative externalities are mostly hidden. Those are good places for targeted regulation. I wouldn't be entirely appeased, but a law about the ability to review all information collected about you from a company and strong controls about the access, sale and use of this information would go a long way towards making me less worried about Google (or whoever) changing quite a bit in the next decade and selling off the information.[1]
Because think about it, how far away are Google, Facebook and the umpteen other ad agencies with complex profiles of you from usurping the credit bureaus?
1: Maybe what we need is an interesting billionaire to buy a lot of personal information on all the U.S. politicians from one of the less public agencies and publish it. I'm sure we would get a law passed in record time.