Abuses of this technology are inevitable, but we haven't seen it yet. It's the source of magical "how did they do that" wow factor in technology that touch screens and thin devices used to have.
> And please - like Google cares that you ordered that book from Amazon. Until they do.
Well, if Google starts caring that I ordered a book from Amazon (more than they already do - Google Now shows me info on my purchases, including delivery time), then they'll do what exactly? Tell their self-driving cars to kill me because I didn't use Google Play?
Google so far has a stellar history of being helpful, pro-user, quite often pro-bono at it. Please apply these levels of scrutiny to someone else first, like every other SV startup running on the investor-storytime model.
For example, a government (China?) may pass a law to force Google to disclose the list of nationals who bought certain books (political book criticising the Chinese government?), and Google may choose to comply to stay in that market.
But basically, you can draw such arguments about anything. What if the evil government asks my local bookstore for CCTV recordings and credit card recipes? What if they ask my bank?
If your government wants to be evil, they will find a way to do this, regardless of whether people posted their data all over the Internet or not. The problem is with your government and not with the tools they would use in a hypothetical, unlikely scenario of going batshit insane in the nearby future. It's like a country deciding to destroy all roads and bridges because they can be used by an invasion force to quickly overrun the country. Well, they would be, but since you destroyed them your enemy will airdrop soldiers on you in the extremely unlikely future when they decide to invade. In the meantime, you have no roads and bridges.
1. getting a warrant and running down to the book shop to ask them about what a single suspect bought, and
2. Having enormous amounts of data collected about hundreds of millions of people, processed and interpreted, essentially sitting in one place. Whether Google voluntarily gives it up or not is almost irrelevant (as we saw, the NSA is happy to tap Google's private fiber without their knowledge even when Google is cooperating on other fronts).
Of course the government will find ways to be evil if it wants (and I think that we're generally lucky because it doesn't seem to want to with any particular intensity). But that's not really the point here.
It's the difference between having many small, complicated little targets that each yield very little information, versus one conveniently enormous target that yields information on everyone.
I structured my previous comment in this way to proactively address the argument that someone always brings up - the story of how Nazi Germany used census data to track down and exterminate Jews. My point being that yes, evil government could use this data to do evil things in an efficient way, but it doesn't mean that we have to proactively stop doing censuses - they have many other, real, actual, positive uses.
Or another example - the best way to prevent a house fire from burning down a neighbourhood is to not build houses near each other. But instead, we invest in firefighters, better materials and procedures, all of which addresses the problem of fire spread. Why? Because we want the houses to be close to each other.