The Supreme Court ruled in Miranda v. Arizona that the police must advise individuals of their rights at that time they are taken into custodial arrest. In effect, at the time they are detained and when police questioning turns from general information gathering of an individual to accusatorial interrogation, that is when the Miranda warnings must be given. The warnings are a product of the Fifth Amendment right against self-incrimination.
In New York v. Quarles, 467 U.S. 649 (1984), the Supreme Court created a narrow exception to the Miranda warning requirement. That case involved a situation where police chased a suspect into a store. They apprehended and searched him, and then found an empty holster. They suspected he had ditched a gun. Before giving him his Miranda warnings they asked him where the gun was, he told him, and then they gave him his rights. The Supreme Court upheld this as valid. They ruled that police could question a suspect to address imminent threat to the safety of public or officer. Once that threat was abated, then the police would need to offer Miranda warnings. The public safety rule is not blanket exception to obtain evidence or testimonial information that may be used against individuals to convict them.
With Dzhokhar A. Tsarnaev, the police were within the Constitution not to read him his rights when they first apprehended him. They needed to determine if he had bomb or other weapons that could threaten police or public safety. Once that threat was addressed t hey should have Mirandized him. It would have taken a few seconds and no harm would have followed form doing that.
But would it not be better not to take a chance. Why risk prosecution by not giving him his rights? Depending on what he is charged with and what evidence there is, this decision could be legally damaging to the government's case.
The second reason why not Mirandizing Tsarnaev is wrong more about the broader concept of democracy and human rights in a free society. After 9/11 the Bush Administration invoked questionable legal doctrines about the rights of the president and the government to bypass the Constitution and international rights in the apprehending, detaining, and prosecution of suspected terrorists. They concocted questionable theories about torture and tried to offshore illegal activities to places like Gitmo in Cuba. (I have written extensively about this issue in my two articles "Defending American Presidential Authority in a Post 9-11 World: Examining the Justice Department Memoranda." Democracy and Society Volume 5, Issue 2 (May 2008) and "Democracy on Trial: Terrorism, Crime, and National Security Policy in a Post 9-11 World,"38 Golden Gate Law Review, 195 (2008).)
Presidents do not have extra-constitutional power. Respect for our constitutional rights is not something we do when it is just convenient. It is something that is required of a democratic free society. It is what separates the US from non-democratic countries. Tsarnaev is a US citizen arrested and accused of crimes in this country. There is no reason to exempt him from the Constitution. In a free society the government carries the burden to prove guilt and it is required to play by the rules. The rule should apply here to.