Tsarnaev’s attorneys are court-appointed. The court no doubt made sure to appoint the best skilled defense attorneys available. They have to work with the case they were dealt, and for a relatively low hourly rate. Not to do everything they possibly can to free or mitigate the charge/punishment to their client would be a serious ethical lapse. This is how the US judicial system works, every benefit conceivable to the charged party.
Tsarnaev’s attorney: