Fourth Amendment to the United States Constitution
"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
It is fairly clearly spelt out and in the digital age our papers/effects are stored online. I can't see it any different if you store your documents/emails at Google than if the founding fathers stored their documents at a bank the above is clear that they can't be searched without a warrant and probable cause.
At some point we have to acknowledge that the Constitution is just another piece of paper that only gives the illusion of protection from those in power.