You make a great point, and I think this topic is explored in greater depth in Hosseini's second novel,A Thousand Splendid Suns.  Inthe Kite Runner, women do not play a strong role; the focus is primarily on the brotherhood between Amir and Hassan and the fragile relationship between Amir and Baba.  In my opinion, Hosseini does not portray women in an injust manner, so I don't see any particular reason for those characters to seek any form of justice.

This is a great question. In my opinion, the book does not really explore the topic of justice for females as a dominant theme. We need to keep in mind that books are limited in scope and so they cannot be all things to all people. With that said, there are a few noteworthy instances, where we see people seeing the injustices of women and standing up against them. 

First, while Baba and Amir are escaping from Afghanistan, Baba stands up for a women who was about to be raped. He even risked his life. For Baba, to treat women in this manner was unthinkable and he would even risk his life to protect them. We can say that Baba was an honorable man. 

Second, we know that Soraya, the wife of Amir, hated the way Afghan women were treated. She had good reason, because she was viewed a certain way, because she had a past with another man. She saw the double standard and hated it. However, there was only so much she could do as one individual. But in the end, she was strong, like her father, the general. 

Third, we see that Amir still marries Soraya and loves her. This shows a step in the right direction as well, because this action could be interpreted as going against cultural norms.