The "Miranda rights" that we're now familiar with from cop shows resulted from the Court's decision. The point is not to confer on a suspect a sudden, emergent right to remain silent. It is, rather, to ensure that anyone facing the duress of law enforcement interrogation — deserved or not — at least understands (or remembers) his basic rights under the Constitution. No American citizen, ever, has to say a thing to the police if arrested; everyone has the right not to "be a witness against himself," as the Fifth Amendment articulates. If someone can reasonably argue he didn't know that right existed, the Supreme Court says that self-incrimination cannot be used.
In 1984, the Court crafted an exception. In cases where public safety was at risk — where, in other words, the police need information urgently to preserve safety — officers may question a suspect. That case stemmed from the arrest of a suspect who was asked where his firearm was hidden. He told the police prior to hearing his rights, but the Court allowed the testimony to be used. The FBI's website clarifies its interpretation of what's called the "public safety exception":
Recent and well-publicized events, including the attempted bombing of Northwest Airlines Flight 235 near Detroit, Michigan, on December 25, 2009, and the attempted bombing in New York City's Times Square in May 2010, highlight the importance of this exception. Those current events, occurring in a time of heightened vigilance against terrorist acts, place a spotlight on this law enforcement tool, which, although 26 years old, may play a vital role in protecting public safety while also permitting statements obtained under this exception to be used as evidence in a criminal prosecution. In brief, and as discussed in this article, police officers confronting situations that create a danger to themselves or others may ask questions designed to neutralize the threat without first providing a warning of rights.
After Tsarnaev's arrest on Friday, U.S. Attorney Carmen Ortiz indicated that the government was invoking that exception to question the suspect. It fits with the Obama Administration's attitude toward the exception. In 2010, it released a memo arguing, as described by The Times, that "the 'magnitude and complexity' of the terrorist threat justified 'a significantly more extensive public safety interrogation without Miranda warnings than would be permissible in an ordinary criminal case.'" This decision, generally seen as an expansion of the application of the rule, has not been tested in court.
According to authorities, the Friday-Monday period of the last week went something like this. On Friday, Tsarnaev was taken into custody. Over the weekend, he was questioned about the bombings and reportedly indicated that he and his brother acted alone. On Monday morning, Judge Bowler arrived and read Tsarnaev his rights, then the charges against him. At that point, according to the Associated Press, which cites "four officials of both political parties who were briefed on the interrogation," the bombing suspect "immediately stopped talking." Fox News reports that the FBI was surprised Bowler arrived when she did.
[S]ources say there was confusion about Bowler's timing, with some voicing concerns that investigators were not given enough time to question Dzhokhar under the "public safety exception" invoked by the Justice Department.
Two officials with knowledge of the FBI briefing on Capitol Hill said the FBI was against stopping the investigators' questioning and was stunned that the judge, Justice Department prosecutors and public defenders showed up, feeling valuable intelligence may have been sacrificed as a result.
At Fox Nation, the Breitbartian arm of the news conglomerate, the headline was more direct: "Boston Bomber Read His Miranda Rights, Immediately Clams Up."