In answer to the posed question, HELL YES! It's not just a "Voluntary" thing, it's an expectation and a responsibility as an American Citizen.
It would be just as legal as being drafted into the Military and "Fighting For Your Country" under conscription. And just as constitutional. After all, you would be fighting to protect all the other freedoms guaranteed you by the US Constitution and the Bill of Rights . . which is what you're sworn to do when you become a member of the Military.
And . . Your responsibilities didn't end the day you were discharged!