Orin Kerr provides an interesting analysis, at the Volokh Conspiracy:
1) A lot of people assume that the police are required to read a suspect his Miranda rights upon arrest. That is, they assume that one of a person’s rights is the right to be read their rights. It often happens that way on Law & Order, but that’s not what the law actually requires. The police aren’t required to follow Miranda. Miranda is a set of rules the government can chose to follow if they want to admit a person’s statements in a criminal case in court, not a set of rules they have to follow in every case.
There are questions of legality and questions or morality. Based on Kerr's analysis, it looks like it's legal to question Tsarnaev without reading him his Miranda rights. But is it moral? I think it is. I don't see the harm in allowing the police and FBI to interrogate Tsarnaev about this attack, if they aren't planning on using his statements against him.