Principles are principles, but they are interpreted in context. It's not about "fleeting public zeitgeist" but about recognizing that American principles have always been about balancing security with liberty. A lot of people would have us read the word "reasonable" out of the 4th amendment, but it's there and it is what implements that balancing. The question is how to find that balance in a world where China can attack important economic assets digitally instead of having to cross an ocean physically.
I should note that I'm not arguing that the government should read text messages. I tend to think that if you're sending something in clear text so some flunkie at AT&T can read it, its no problem if the government can too. My point is that you can't just ignore the real security concerns. The U.S. has never been about liberty at the cost of security.
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.
I should note that I'm not arguing that the government should read text messages. I tend to think that if you're sending something in clear text so some flunkie at AT&T can read it, its no problem if the government can too. My point is that you can't just ignore the real security concerns. The U.S. has never been about liberty at the cost of security.