it might be a low magic setting for players
Stradh, lord soth, any other darklord or the mist itself, the dark hunt
all this aint exactly low magic :-), neither is the dark powers or the mist .
hell there are even skeleton archers with negative damage, with strong negative plane ties both the +1 and negative damage in my eyes are quite fitting.
dont think of it as +1 bows they use but rather as an ability tied to the negative plane, enhanced by ravelloft like arcane archers +1 arrow power