Why did Howes v. Fields rule that a prisoner was not automatically in custody for Miranda purposes?

In Howes v. Fields (2012), the U.S. Supreme Court ruled that a prisoner is not automatically considered to be "in custody" for the purposes of Miranda warnings simply because they are incarcerated.1

The Court determined that the standard for "custody" requires more than just the fact of imprisonment; it necessitates an assessment of whether the circumstances of the interrogation would cause a reasonable person to believe their freedom of action was curtailed to a "degree associated with formal arrest".1

In the case of a prisoner, the Court reasoned that because they are already in prison, the interrogation does not necessarily impose the same "added" restraint or coercive pressure that Miranda was designed to mitigate in a typical police-station setting. Consequently, when a prisoner is removed from their cell and questioned about events that occurred prior to their imprisonment, the absence of the specific coercive environment associated with a formal arrest means that the protections of Miranda are not automatically triggered.1