Realistically it doesn't, programmatically it does: We have ambient light already. If you made an enclosed room, shadows would be cast into the room, however if it was large enough the shadows would peter out and the ambient light would take over in the center.
The answer: Play on maps with no ambient light and do all your lighting yourself.
I thought light was controlled by a universal vector, and in this instance, an object that blocks the vector from touching the ground, therefore creating shadows. If this were a large room, then wouldn't it not light the very middle since shadows travel over the center of it, since light vectors come from infinity in a certain direction, then light cannot be created without a source.
I may be wrong in how the game lights, but this is my assumption.
