As to the topic at hand, I find it interesting that the question isn't "Should google be targeting non-subscriber email ads", but whether or not they should be looking at ANY email content.
Email fundamentally requires computers to read your email. Your email client sends an RFC 2821 formatted message to an SMTP server, and that server reads your email content. If it didn't read the To: header, it wouldn't be able to send the message to the right destination. If it didn't read the body, it couldn't pipe the body out to file storage or out another SMTP/IMAP/whatever connection to the recipient. Being concerned about computers reading your email is like being concerned that your mailman is driving your paper mail around in a truck. It's fundamental to the process.
If you say, sure a computer can read it for one reason but not for another, then you're saying you think you ought to have the right to control what software consenting adults write for each other. I'm against you having that power over me.