Google argues for right to continue scanning Gmail

I don't think it's quite the same.  The post office is scanning the envelope, they're not opening the mail and scanning every word of the letter.

If it were purely in the context of them seeing who is emailing who, then I would agree with you, but this is a lot deeper than that.

I don't have a huge problem with what they're doing personally because there haven't been any examples of them mis-using the data.  However, I also recognize that there's an interesting legal question that needs to be answered.  Fedex is a company that would be analogous to google.  People use them as a private entity to send their mail.  Fedex isn't allowed to open a package and read the letter for marketing purposes or anything else.  So, with email being digital it's obviously easier to do but it's the same legal argument.

True, the system is ripe for abuse.

I think purely in the context of them having algorithms that parse data to give ads, it's easier to justify.  But, there's nothing that says that's all they do.  It's probably safe to assume an engineer working at google can go through emails and read them for "testing purposes".  What if that person he's viewing just so happens to be his wife that he's going through an ugly divorce with and uses the information against her.  Now the legal argument gets a little deeper because you can either view the contents or you can't.  You can either open somebodies snail mail or you can't.