Any webmail service that stores email unencrypted on its servers is subject to subpoena / FISA requests, and there isn't much they can do about it.
They could roll out client-side encryption -- but there's little business incentive for them to do so on free accounts. That said, as an option for paying apps users, it would be quite valuable.
Automatic scanning of emails isn't ethically suspect in my opinion.
My reasons for not working at google right now are: lack of desire to engage in big company politics, the apparent softening of the 20% time policy, regular-corporate/long hours, and the commute I'd have to take. Otherwise it'd be near top of my list should I go back into full time business.
In terms of "big companies", their sins are relatively innocuous, and if I wanted to work in big tech they'd be #1 on my list given the technology & resources that they make available. I'd certainly never work for Apple, Facebook, Oracle or Microsoft. I'd need a lot of convincing to work for Amazon.
But when you think of it for like 2 seconds, automated scanning does help the NSA. Such software is easy to re-purpose. With the right subpoena, I give Google a week before it gives away a nice social graph of US dissidence to the NSA.