I would think that emails are a lot more personal and conversational than the bills and coupon flyers that one receives in their regular outdoor mailbox.
If I were to make a comparison, I would compare sharing an email to having your spouse stay on the other line while your in the middle of a phone conversation.
Now, that isn't to say that they COULDN'T do that. But I figure that's the reason that some people have for giving their spouse the password to their email, so that they can go in and read your emails at their own convience (and curiosity).