Would all of our lives be safer if the government could break down all the doors it wishes, listen to all the conversations it could find and read whatever emails and text messages it could acquire? Perhaps. But who would want to live in such a society?
To prevent that from happening here, the Framers ratified the Fourth Amendment, which is the linchpin of privacy and was famously called by Justice Louis Brandeis "the right to be let alone — the most comprehensive of rights and the right most valued by civilized men." He wrote those words in his dissent in the first wiretapping case to reach the Supreme Court, Olmstead v. the United States, in 1928.
Roy Olmstead had been convicted for bootlegging on the basis of words he used in overheard telephone conversations. Because he had used a phone at his place of work that the government had tapped without breaking and entering his workplace, the high court ruled — despite the fact that the government had not obtained a warrant — that he had no right to privacy. Brandeis dissented.
Over time, the Brandeis dissent became the law. The Fourth Amendment, which protects the privacy of all in our "persons, houses, papers, and effects," was interpreted to cover telephone conversations and eventually emails and text messages. So today, if the government wants information contained in those communications, it needs to obtain a search warrant, which the Fourth Amendment states can only be given by a judge — and only upon a showing of probable cause of evidence of a crime contained in the communications it seeks.
If the government does not obtain a search warrant and listens to phone conversations or reads emails or text messages nevertheless and attempts to use what it heard or read to acquire other evidence or directly in the prosecution of a defendant, that is unlawful. That type of information is known as the fruit of the poisonous tree.
Evidence procured that is the fruit of the poisonous tree has been inadmissible in federal criminal prosecutions in the United States for the past 100 years and in state criminal prosecutions for the past 50 years.
Now comes the super-secret court established by the Foreign Intelligence Surveillance Act, reaffirmed by Congress last year under the so-called USA Freedom Act. Beware the names of federal statutes, as they often produce results that are the opposite of what their names imply; and this is one of them.