If cabin lights are dimmed during night landing so that your eyes will get used to the darkness, why do they let you use the reading lights?
That is a good question. It is most likely a compromise. Turning down the cabin lights allows for night vision adaptation for most. Its better to have at least some of the passengers adapted, than none at all. Its a compromise between optimal safety (night vision adaptation) vs customer service (being able to finish reading your Harry Potter book).