The problem with spying on clear text messages is that there's no indication to the user that they have no privacy. When the average person is texting their friend, their phone makes it look like a private conversation. It doesn't say, "Send message to Joe, the NSA, AT&T, and your neighbor's pet dog." It just says, "Joe."

Also, this is one rare case where I would accept reasoning by analogy with a physical analog: mail. People have an expectation of privacy when sending letters, and they should rightly expect e-mail and text messages to be treated similarly.