Defensive weapons and offensive weapons are separate so that defensive weapons can be cheaper than offensive weapons. This gives the defense an economic advantage in warfare, and deters reckless military aggression. Honestly, I think the game would be more interesting if there were no separate categories for defensive and offensive weapons. Players would then be forced to rely more heavily on their diplomatic judgement to ascertain whether or not other players posed a threat to them, instead of merely relying upon the advantage which the game provides in purchasing defensive weapons.