So cogently argues Reason Contributing Editor Julian Sanchez over at Cato's blog. The heart of the why:
under the modern understanding of the Fourth Amendment, the Constitution is concerned only government actions that violate a "reasonable expectation of privacy," which courts have generally understood as limited to the exposure of what was previously secret. When we entrust sensitive records to third parties—be they banks, Internet Service Providers, or other members of our churches or political organizations—we "assume the risk" that they will reveal the information to the government, according to the courts' logic, and so waive our expectation of privacy.
….Yet a growing number of investigative techniques—from GPS location tracking to DNA analysis—allow the government to conduct an intuitively troubling degree of monitoring, potentially on a vast scale, by targeting information that is at least in some sense "public."….
…the root of the trouble with current Fourth Amendment doctrine is the very idea that the prohibition on "unreasonable searches" must be viewed primarily through the lens of privacy. If we consider public surveillance camera networks, or some recent cases involving "dragnet" location tracking by law enforcement, I think we find that whatever intuitive unease we feel about the methods employed has less to do with a sense that the individual "right to privacy" of any particular person has been violated than with concerns about the government monitoring the citizenry as a whole in these ways…..
An alternative approach…is offered by Yale's Jed Rubenfeld in his article "The End of Privacy," which I wrote about last year. Rubenfeld's Big Idea is that we have ignored the crucial role of "security" in the Fourth Amendment. We're now accustomed to arguments over the "tradeoff" between the competing values of "security" and the "privacy" protected by the Fourth Amendment, but by its own terms, the Fourth Amendment stipulates that "the right of the people to be secure…against unreasonable searches and seizures, shall not be violated." We tend to read this, in effect, as simply saying that the right against unreasonable searches and seizures shall not be violated—so that the words "people" and "secure" don't end up doing any real work. But as Rubenfeld notes, "security" was actually a significant legal concept in the minds of the Framers—something free people enjoyed by contrast with the insecurity generated by arbitrary and discretionary government power. ….
To think of the Fourth Amendment this way—as not exclusively about privacy, but about "the right of the people to be secure"—is necessarily to take a more architectural view of its protections. But Rubenfeld offers something closer to an applicable test: Rather than asking whether an individual reasonable expectation of privacy has been violated, we ask whether people would remain secure in their liberties if a particular search method were pervasive. If it would not, we ask what restrictions—such as requiring a probable cause warrant or "specific and articulable facts"—would sufficiently narrow the method's application so as to leave reasonable citizens secure…..
On the currently dominant view, then, the government violates privacy (and the Fourth Amendment) just in case it performs actions that would be privacy violations if conducted by anyone. Yet the Framers had good reason to be particularly concerned with the social implications of government information gathering. Those concerns had less to do with "privacy" as such than with the structural balance between personal autonomy and state control…
Sanchez thought at length about the evolving meaning of the Fourth Amendment in the face of techological change for Reason magazine in this great January 2007 feature.