Immunity of noncombatants

Catholic moral theology teaches the immunity of noncombatants. But in virtue of what are they immune? From my reading, "innocence" is frequently the condition that confers immunity. Noncombatants are immune because they are classified is innocents, and it's wrong to target innocents. Wrong to intentionally take innocent lives. (Although "intention" is a term of art, viz. double effect theory.)

But in what respect are noncombatants deemed to be innocent? Let's consider some possibilities:

1. Morally innocent

It might be a moral category. To be morally faultless or sinless.

i) If so, the principle is too strong. In traditional Christian theology, human beings are sinners. Even children are subject to Adam's sin. Even children have evil impulses. 

ii) Perhaps in the case of children before the age of reason, it might be said that they are inculpably evil because, despite their evil impulses, they lack the intellectual capacity to be consciously evil. 

iii) But even if we grant that distinction for argument's sake, the class of noncombatants in just-war theory is far larger than children before the age of reason (which has fuzzy boundaries in its own right). 

2.  Judicially innocent

It might be a legal category. To be blames or guiltless in relation to criminal wrongdoing. 

i) But it's unclear why that would be the principle. If you have combatants on both sides, where one side is fighting a just cause while the other side is fighting for an unjust cause, you might argue that the unjust combatants forfeit immunity due to their legal complicity in an unjust war, making them liable to death or injury as a just desert. But the same reasoning can't apply to just combatants. By that logic, immunity extends to just combatants. 

ii) By that criterion, moreover, civilian policymakers who wage unjust war ought to forfeit immunity and face the same fate as combatants. Likewise, civilianswho provide necessary support services, viz. munitions factories.

3. Harmless

It might be a pragmatic category. Most civilians don't pose an imminent danger to the opposing nation or troops. They enjoy immunity because they are innocuous or nonthreatening compared to combatants. 

Certainly that captures an intuition regarding the immunity of children (with rare exceptions). However, the principle seems to lean on a rather artificial dichotomy regarding the immediacy of the threat. A military engineer may not pose a direct threat to the opposing nation or troops, but the direct threat may depend on his invention of military technologies. Likewise, take the policymakers who foment war. Or civilians who construct missiles, tanks, bombs, bombers, &c. Although there's a sense in which they are personally innocuous inasmuch as they don't shoot guns, they conscript combatants or supply combatants. 

I think each of these principles has some merit in limiting the scope of legitimate targets, but they're inadequate, either in separation or combination, to justify the absolute immunity of noncombatants across the board.