Actually, it seems to me that Google's point is a good one.
If somebody sends an email, they are normally letting the recipient do whatever they want with the email, including forwarding it to third parties, publishing it in a newspaper, or indeed, letting Google read it.
I believe that in general, you make no assumption when sending a letter to someone that no one else has the right to read that letter. If the recipient decides to publish the letter, he publishes it.
In fact, only governments (ha!) seem able to restrict the right of the recipient to publish their letters. This is precisely the criticism that has been leveled at National Security letters and "super injunctions".