"Enemy combatant status" for an American citizen on American soil might be legal, but it is not Constitutional. If the charges are proven true in a court of law - and the evidence appears overwhelming - Tsarnaev will probably be executed just like Timothy McVeigh was.
We have a Constitutional process for dealing with American citizens who commit criminal acts, and our Founders wisely built safeguards to protect every American citizen from being convicted and punished without a trial. What is convenient, expedient and popular is not always right. History shows that what a government and a public holds to be popular can change in a hurry, and those who have shortcut due process for fellow citizens have later found their own due process ignored.
Here is the oath that this "Bozo" has sworn many times:
"I do solemnly swear that I will support and defend the Constitution against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same... So help me God."
Naturalization can be revoked. 8 USC § 1451(c), 8 USC § 1424(a)(4) among others.