We not only shouldn’t hold Tsarnaev as a combatant, we actually can’t.
Under the AUMF as interpreted by the courts, and under the NDAA as passed by Congress, the administration is authorized to hold in military detention only those who are “part of” or “substantially supporting” Al Qaeda, the Taliban, or associated forces. Nothing that has come to light publicly has shown that Tsarnaev was operating as part of any group covered by the AUMF.
(And I’m confused about the anti-Miranda crowd: not Mirandizing doesn’t take away his right to remain silent, it just doesn’t remind him of it. I have trouble understanding the so-called advantages of waiving it.)