"They believed they needed more time. This is not a good way to stop another bomb from going off," Rogers said, according to the Journal.
Investigators had invoked a public safety exception to put off reading Tsarnaev his Miranda rights, including telling him that he had a right to remain silent. Usually, prosecutors can't use statements made without a Miranda warning against a suspect in court. This exception lets investigators continue questioning a suspect and later use that information against the person in a court of law.
The idea behind the exception is that investigators feel free to question suspects about imminent risks to the public. Tsarnaev stopped speaking as soon as his rights were read to him.