If I tell you I murdered someone, should the government have to get a warrant to get that information from you? What if they asked you if I visited your bookstore last week? What if you kept a journal indicating each time I visited your store, or bought a book? What if they asked Amazon for a list of when I visited their site, or bought a book? (Keep in mind Amazon has the right to refuse to give that information without a court order...the question is whether I can protest that subpoena, or only Amazon can).
At some point, there needs to be a line drawn. The 4th amendment has been interpreted to protect "reasonable expectations of privacy" in order to draw that line. If I tell you I murdered someone, or if you tell the police when I visited your store, that's over that side of the line. I have no reasonable expectation of privacy. I freely gave up this information to a third party.
The copyright law has an entirely different history, text, and history of interpretation.
I agree that the third party doctrine is no longer workable in the digital age (people probably have just as much of an expectation of privacy in their DropBox as their C: drive), but I don't think the other viewpoint is entirely unreasonable, especially during the pre-digital age.