There's a fairly lengthy theological tradition of describing Christians as "soldiers" for God.
(Battle Hymn of the Republic, anybody? "Mine eyes have seen the glory of the coming of the Lord; He is trampling out the vintage where the grapes of wrath are stored; He hath loosed the fateful lightning of His terrible swift sword; His truth is marching on..."
Or Onward Christian Soldiers? "Onward Christian soldiers, marching as to war, with the cross of Jesus, going on before...")
My guess is that "warriors" just sounds cooler than "soldier" to the people making that description. And with the allegiances between the Christian right and the Bush administration and the war in Iraq, there's probably an intentional attempt to connect average Christian with the war.
I think it's a crappy, crappy metaphor, BTW, and dangerous and wrongheaded when taken literally. But hey, I'm only a Ph.D candidate in theology, so what do I know?