Um, common sense I guess. I've always played that if a weapon has an alignment (in this case good) then it bypasses DR. How else would a weapon bypass DR/good?
I know that the align weapon spell works, but isn't that because it give the weapon a temporary alignment?
from prd...
Align weapon makes a weapon chaotic, evil, good, or lawful, as you choose. A weapon that is aligned can bypass the damage reduction of certain creatures. This spell has no effect on a weapon that already has an alignment.