[voice]
A soldier who uses free will is one who commits treason against his country. One may have the free will to become a soldier, but to be one must have choose to give up the free will that others so covet in order to preserve it for the common citizen.
So yes, you could say that one uses their free will to become a soldier, but to be a soldier, one also chooses to become a subordinate, putting his life in the hands of his superior and gives up his right to choose for himself.