I think that asks a bigger question: Why shouldn't we expect it? Seriously.
So, you are saying that in my example of a hand to hand delivered postcard or post-it note, that you would expect privacy and security? You would not expect anyone along the chain to read it, and funny looks from a prudish neighbor that was part of the delivery chain would come as a complete surprise and shock to you, and you would expect the $10,000 in cash to remain perfectly safe, and feel no need to change the pin on your credit cards?
I don't think the point is what you would expect, in terms of reality today. It's what you should be able to expect. And I don't think that, all things being equal, you should have to lock the virtual door in order to get privacy. Privacy shouldn't have to be based on security. Because if it is, then we have no privacy. There are always people that with the appropriate amount of effort and desire, that can crack any security.
It doesn't matter what you expect. In a broken system, if you know it is broken but aren't willing to admit it, how can you ever expect to come up with a solution to the problem? Pointing fingers of blame at those that point out that there is a problem, isn't a solution. In other words, pointing fingers of blame at Google for stating that there shouldn't be an expectation of privacy, doesn't fix the root problem that caused them to be able to state that.
And even if you can still reasonably expect privacy, that nobody will enter when you leave your front door unlocked, don't expect your neighbors to look the other way if you refuse to put up shades or curtains and stand nude in front of the big picture window.
Expectation may be one thing. But using expectation as a justification or an argument for the inevitability and necessity of a certain behavior is another.
To my mind, part of your argument strays dangerously close to the tendency of some to blame the victim.
Just because something bad happens is not the same thing as establishing that it must happen...
Admitting and pointing out that the system is broken is not blaming the victim that is forced to use the broken system. Not at all. And if it seemed as if that was what I was trying to do, then you misunderstood, and I apologize, because that was not my intention at all.
I would very much like to see the system fixed, or completely thrown out in favor of something better. But that is never going to happen if people are too busy running around blaming others instead of coming up with a solution that is easy enough for everyone
to use. Whether that means adding easy to use, idiot proof PGP (or alternative) support to all
email clients and webmail services, or throwing the whole thing out in favor of something like bitmessage
(or something similar) then so be it. Just hurry up and fix it already because I have a business to launch that is going to depend on idiots being able to send me sensitive information, securely, for which I currently have no free easy idiot-proof cross-platform solution. There is nothing out there that I can say "use this" or "install this" and not have to worry about providing many hours of troubleshooting and support to get them set up and using it. If I have to do that, I'll have to raise my prices, which I don't want to do. And if I say "screw the idiots" then I'll lose a lot of business, because my target clients are the idiots, the lazy, and the insanely busy, and I expect many of my clients to be a combination of 2 or more.