The problem is not serverload or something. The problem is, that your client software can't display anthing different than what was ppre rendered.
So if you add a red light to you appartment, then the game adds a red light mask to your textures. But this method is only possible for small areas, as your rooms/shops/malls are all seperated areas (you know the lag when entering/laving these areas).
If you would apply a whole new lighting map to an LA, it would make your game unplayable. The servers would just send the command: new daytime, but your machine cannot handle it, as this can't be done with usual game graphic cards in a resonable time.
This is the reason why most games dont have dynamic lighting.