It has, after all, according to the Bush definition of the term, any person who fires at a US soldier is a terrorist.
Well, yes... but...
Firing on the military is an act of war. It's pretty simple when another state does it; it's war, end of story. But individuals firing on military targets aren't acting in the interests of a state, so how does one declare war on individuals? Is it possible? What rights do those individuals have?
See, it's much easier to simply label those individuals as terrorists, as it makes for a smaller can of worms, and allows Congress to shirk its responsibility of calling a war a war.