I asked the question about barbarian melee units getting weapon promotions in the Bugs thread without a response.
I opened the World Builder and although they had Bronze Weapons, they had no copper mines in their cities. Same for later with Iron Weapons.
It appears to me the barb melee units are getting the weapons 'automatically.'
It's a shame because it would create a fun quest to hunt down and take out barb metal mines without having to take out barb cities and move the counter. This should stop the metals promotion.
As it is now, it appears they spawn with the promotion automatically.