Privacy is created by social norms. It's no technical challenge for me to borrow your paper mail from your mailbox, steam it open, read it, copy the bits I find interesting, seal it up again, and replace it in your mailbox. But, in doing so, it's understood that I'm doing an awful thing. It's so awful that it's against the law:
http://www.wbrz.com/news/postal-workers-accused-of-tampering...
... and, all else being equal, juries will not be inclined to sympathize with me.
Similarly, it's somewhere between very rude and illegal, depending on circumstances, to intercept or interfere with someone's email. If you happen to glance at someone's email you're expected to keep politely silent about it, as you would be if you happened to glimpse your neighbor through a window of their house. You're certainly expected, under pain of felony charges, not to tamper with or forge someone's email, just as you're expected to avoid entering your neighbor's house without knocking even if the front door is standing open.
Google, on the other hand, seems to be constantly trying to establish the precedent that it's perfectly normal and polite for any aspect of your life - currently including, but presumably not forever limited to: the state of your front yard, the contents of your photo album, the list of movies you've watched on YouTube, and the contents of your mailbox - to be sampled, data-mined, correlated, and archived forever by entities completely outside your knowledge or control so long as those entities are using secret algorithms to do it.
If you'd tolerate this behavior in a friend, you may by all means continue to have Google as a friend. I, however, am getting increasingly uncomfortable with Google sitting in my living room, and am increasingly tempted to escort them politely but firmly to the door and then deliberately misplace their address.