Now the news comes that the person who actually read the Miranda warning to Tsarnaev wasn’t even an FBI agent, but a U.S. magistrate judge (magistrate judges are sort of like junior federal judges — they are appointed by the courts to assist them, but they are not real judges, and are subject to revision by real federal judges).
This is an outright violation of the separation of powers. It is not for federal judges, or worse yet their assistants, to rove around looking for criminal cases in which to act as law enforcement agents. The decision whether to read Miranda lies up to the executive branch. The right of the courts to affect the warnings and conditions of interrogation stems only from their control over the criminal trial of the suspect. Miranda itself is only a declaration by the courts that they will exclude from evidence any confessions received without a warning. Under the Constitution, the President is responsible for the enforcement of the laws, not the courts — the courts’ constitutional job is to decide cases and controversies that arise under those laws.