so how would you determine who was involved in the war against you? Just because some weren't involved doesn't suggest all weren't, and if you assume so, you'd be wrong and stupid for sparing enemy combatants.
think of it like this, not all snakes are venomous. Some can bite you, and it wouldn't be different to me biting u in that they pose no threat to u, and other species of snakes can kill you with a half a bite. if you don't know which species are venomous and which aren't, would you give in your hand and take a bite to find out? No!
Part of the reason why you take slaves during wars is to make sure kill to any problems in the future, by taking their women and men as slaves, you can be sure you'll be safe from them in the future. refer to the snake anology and you'll know what I am talking about my nigga.