This is bullshit. Draco deserves at the very least 5-10 and Narcissa a minimum of 15-20. And he can still pass the trial for not sending them to rot in Azkaban...
This was good judgement Lucius permanent askaban his wife 5 years in jail for helping due to loyalty (bad things are still bad) and Draco gets a proper punishment for having not much of a choice in choosing his own life with his upbringing (it is not easy to truly walk out on your family)
