The idea that theft is the "ultimate sin" does not actually come from Afghan society in The Kite Runner by Khaled Hosseini. It comes from Baba's personal code of conduct, which he teaches Amir. He says this in chapter three:
"Now, no matter what the mullah teaches, there is only one sin, only one. And that is theft. Every other sin is a variation of theft. Do you understand that?"
Note that Baba, the man who makes and lives by his own rules in any area of his life, in this statement has circumvented anything Amir's religious leaders might have taught his son. This idea that theft is the greatest sin comes purely from Baba's philosophy of life. He continues:
"When you kill a man, you steal a life," Baba said. "You steal his wife's right to a husband, rob his children of a father. When you tell a lie, you steal someone's right to the truth. When you cheat, you steal the right to fairness. Do you see?"
This is Baba's reasoning, and he lives by it--or at least he lives by it when it is convenient to him, because of course we learn that Baba had an affair with his best friend's wife and that Hassan is Baba's son, something Baba lied about for his entire life. He stole Amir's right to the truth when he lied by omission.
But Baba is not finished with his moral (and hypocritical) instruction:
"There is no act more wretched than stealing, Amir," Baba said. "A man who takes what's not his to take, be it a life or a loaf of naan...I spit on such a man. And if I ever cross paths with him, God help him. Do you understand?"
Even though this is a Baba philosophy rather than an overruling Afghan philosophy, it does have an impact on the events of the novel. Amir is having trouble dealing with his conscience after running away from Hassan during Assef's assault. Knowing how his father feels about stealing, Amir plants some money and his new watch under Hassan's mattress and accuses Hassan of stealing them. He knows Hassan will be compelled, by love for Amir, to lie and say he did it; Amir also knows Baba will feel compelled, by his principles, to send Hassan away for stealing.
This concept of theft as the ultimate crime is the foundation of Baba's expectations of others, though he does not feel compelled to follow his own laws when it is inconvenient. This philosophy is the cause of trouble for both Amir and Hassan throughout the novel.