Trying Dzhokhar: Two (apparently) good calls
With no established ties to a terrorist organization currently at war with the United States, Tsarnaev would not meet the current legal definition of an enemy combatant. Being a Muslim who detonated bombs in a U.S. city is not enough.
It is conceivable that the investigation of Tsarnaev would reveal connections that would make him eligible for enemy combatant status. Apparently the President believes that such connections are highly improbable. Either that or Obama wants to get Tsarnaev into civilian court before such connections can be established because the President has a strong distaste for using military courts even when their use is appropriate.
The President absolutely made the right call, though, in using the public safety exception to delay Mirandizing the suspect. This case clearly falls under that exception.