The EFF has a good page on the principles underlying "reasonable expectation of privacy." See: https://ssd.eff.org/your-computer/govt/privacy.

The crux of the matter is "knowing exposure to third parties." So you have an expectation of privacy in sealed postal mail, but not say post cards or anything printed on the outside of envelopes. While a postal service as a matter of course can't read what's in peoples' sealed letter mail, e-mail is sent in plain text and can be seen by any intermediary SMTP server as well as system administrators of the sending and target mail servers. That's just the nature of the protocol.

Moreover, the fact that the U.S. Postal Service is an agency of the government puts it under heightened scrutiny as compared to e-mail providers who are private parties. The fact that people would be horrified if the U.S. Postal Service were tearing open envelopes and scanning mail to send targeted catalogs, etc, to people but accept Google, etc, doing the same thing as a matter of course really cuts at the knees of the argument that people have the same "expectation of privacy" in their e-mail as they do their letter mail.

So would an attachment to an email (say a zipset of text files) fall under the category of reasonable expectation of privacy?

Is that attachment encrypted?

But now that gmail has ssl access, this is no longer true, right?

Gmail has SSL to Google's e-mail server. If you e.g. send a mail to someone using Outlook, it's sent in clear text. And at both ends it's scanned to deliver everyone involved targeted advertising.

"Reasonable expectation of privacy" doesn't have an implicit "only as against the government." If you're knowingly exposing the contents to Google and Microsoft to scan, you can't claim to have a "reasonable expectation of privacy" in the contents.

And this is why you always use something along the lines of PGP end to end.

The problem is this works only with 2 parties that have already agreed to, and are willing to use, encryption. How many websites, services, and other sources of email have an option to encrypt your email to you with your public key? Not many at all.

I've stopped putting my public key on emails I send, because almost no one ever encrypted email to me, unless they were specifically sending something perceived to be sensitive.

Give me your key and I'll send you one ;)

PGP needs to be more easily accessed by non-tech folk.

Absolutely. Some of the biggest failures in security is lack of ease of use by and large, in addition to ignorance. I think most people are willing to do the proper thing, but they'll actually do it only if it's easy.

Is it reasonable to have different feelings about an automated system than some random person?

The ad-bots at Google aren't known to be very gossipy.

(I don't mean this to be snide. I think this is an instance of a very interesting phenomenon, where we reason about our society as if it were a village.)

The law avoids anthropomorphizing computers. So the "person" here is Google, not the ad-bots. The ad-bots are just tools used by the person who has access to your e-mail.