To slightly expand on Springheel's (entirely correct) answers:
We have ambient light in dark areas because the object of this game is to hide in shadows, which means it is not feasible for the player to have to rely on a light source like the D3 flashlight. Although the Dark Mod player does have a hooded lantern, if you take this out you become extremely visible to all AI, so you can only use it when you are alone. While there are games which avoid ambient light and make caves completely dark, these are not typically stealth games in which the player needs to avoid emitting any kind of light.
If Dark Mod missions were completely dark, then you'd either be looking at a completely black monitor (as Springheel says), which would be pointless and entirely un-fun, or you'd have to carry around a light source and alert every enemy you see, which defeats the object of a stealth game and turns it into a (clunky and hopeless imbalanced) hack-and-slash adventure.