It is absolutely fair as all the rules of Islam are fair with out fail. Those who are linked with the greatest human being, Prophet Muhammad (SAWA) are gifted more reward when they do good and implement his teachings , and this gift is for the sake of respect and love of the Prophet Muhammad (SAWA). Similarly if they do wrong and violate the message of Islam, they will entitled to double chastisement because of the disrespect they showed to the Prophet (SAWA).
In Quran said : O wives of the Prophet; any one from you commits a proven sin will be punished double, and any one from you submits to the will of Allah and His messenger and does good, we shall grant her double of reward. (33:30 and 31).