The NSA sits at the nexus of violations of both the Fourth and Fifth Amendments with a legal dodge called Parallel Construction.
Parallel Construction is a technique used by law enforcement to hide the fact that evidence in a criminal case originated with the NSA. In its simplest form, the NSA collects information showing say a Mr. Anderson committed a crime. This happens most commonly in drug cases. The conclusive information is passed to the Drug Enforcement Agency (DEA), who then works backwards from the conclusion to create an independent, "legal" body of evidence to use against Mr. Anderson.
Example: an NSA email intercept shows our Mr. Anderson received a Fedex package with drugs, which he hid under his bed. The DEA takes this info, and gets a search warrant for the Fedex data, which leads them to Mr. Anderson's apartment. A new legal warrant authorizes a search, and agents "find" the drugs under the bed right where the NSA said they were in the first place.
Some may call this little more than illegal evidence laundering.
Some Constitutional Background
The Fourth Amendment to the Constitution protects Americans against unreasonable and unwarranted searches. The Supreme Court has generally held that searches of, for example, someone's home, require a warrant. That warrant can be issued only after law enforcement shows they have "probable cause." That in turn has been defined by the Court to require a high standard of proof, "a fair probability that contraband or evidence of a crime will be found in a particular place." The NSA pulling information out of the cyberspace ether bypasses and thus violates the Fourth Amendment.
The NSA violations of the Fourth Amendment enable further DEA and other law enforcement violations of the Fifth Amendment, specifically the critical due process clause. The concept of due process dates back to the 13th century Magna Carta.
Specifically, the use of information obtained illegally and whose ultimate source is concealed from the accused violates procedural due process. This is the requirement that before any government actions to take away life, liberty or possessions, the persons affected have the right to defend themselves, to understand the evidence against them, and to question and call witnesses in rebuttal, one's "day in court." In short, procedural due process aims to protect individuals from the coercive power of government by ensuring that adjudication processes are fair and open.
DEA is blunt in a document released via FOIA as to how conveniently parallel construction violates these rights:
Our friends in the military and intelligence community never have to prove anything to the general public. They can act upon classified information without ever divulging their sources or methods to anyway [sic] outside their community.
Why Do This to Americans?
With exceptions, courts have held that evidence obtained illegally cannot be used in trial. So why bother to fight for an exception when, using NSA data surreptitiously, evidence can subsequently be obtained cleanly under a warrant, albeit a warrant issued by a court kept ignorant of the source of the underlying information. Another reason to use parallel construction is to hide the NSA's role. Apart from the broader goal of not disclosing to the American people what their government is doing, blurring the trail back to the NSA gets around any courtroom attempts that require such data to be shared with the defense. And of course the defense can't ask for something it does not know exists. Lastly, if defendants do not know the ultimate source of the information used to convict them, they cannot know to ask to review potential sources of exculpatory evidence- information that could reveal entrapment, mistakes or biased witnesses.
Needless to say, using information obtained already pre-packaged from the NSA makes DEA's and other law enforcement agencies' jobs much easier. They have to do little work on their own to gather the data needed to track down Americans they seek to prosecute. It's all in the bag.
DEA as the Nexus
DEA seems to be the center of the NSA distribution network, as the program originally started as a way to bust foreign drug dealers before it metastasized into the currrent tool for broadly evading the Bill of Rights.
How widespread domestically is the practice of parallel construction? No one knows. It is known that the unit of the DEA that distributes the NSA information is called the Special Operations Division (SOD.) It partners with two dozen other agencies, including the FBI, CIA, Internal Revenue Service and the Department of Homeland Security. Once laundered of any NSA fingerprints, what those multiple agencies do with the data, and how far they themselves spread it to even more agencies, or to local law enforcement, is unknown.
Why it Matters
There have been complex questions raised about the hiding of NSA-obtained information used to convict Americans, leading to the Solictor General of the United States lying to the Supreme Court about how the Justice Department was not notifying defendants in situations when warrantless surveillance had led in turn to a wiretap order that produced evidence used in court. The Justice Department has taken to notifying some defendents that information obtained via warrantless survellience is being used against them, allowing for a likely Supreme Court challenge. The Justice Department has previously blocked Supreme Court challenges by hiding how information was obtained, thus denying the accused of "standing" in the Court's eyes.
As part of the response to such government actions, organizations such as the Los Angeles County Bar Association are now offering for-continuing-education-credit tutorials to defense attorneys under titles such as "Criminal Prosecutions and Classified Information."
A lot of attention Post-Snowden has been paid to what the NSA does-- vacuum up emails, listen in on Skype chats and so forth. Too little attention has been devoted to what is done with the information NSA collects. The appetites of law enforcement agencies in Post-Constitutional America are bottomless, and the NSA holds terabytes of data to fill them.