There is no Constitutional right to have a lawyer during questioning, only a Constitutional right not to have any statements you make during such questioning introduced at trial. Since they have ample other evidence by which to convict Tsarnaev without using any such statements, there is no particular reason to Mirandize him. We can just accept that the statements made without advising him of his rights are not admissible in court.
See this excellent summary by Orin Kerr for a bit of explanation of how it actually works (as distinct from how you or I or him believe it ought to work). You can also read the Supreme Court's decision in Chavez v. Martinez, 538 U.S. 760 (2003) directly, if you prefer,