to further explain: builtInWeaponSets work much like builtInWeapons, but multiple sets can be defined. one set will be picked at random according to the alien item levels table. effectively this means you can have terror units upgrade over time, just like everybody else, simply by defining 3 sets of weapons for them.
both builtInWeapons and builtInWeaponSets will work on civilians as well as xcom and alien units, but xcom units won't get a randomized weapon set.
this is by because the player upgrades their equipment via research, not random chance.
as for the AI, the civilians and aliens now use the same functions and routines, the only differences are the faction flag checks, and civilians can't take part in reaction fire.
again, this is by design, given that civilians never DRAW reaction fire, it seemed a little unbalanced to allow them to perform it.
if you want some sort of in-lore justification, keep in mind that xcom recruits are supposedly the best of the best that the world's military has to offer, so it stands to reason that the general populace may not have the conditioned reflexes or training required to perform such an action.