Why does Amir call his birthday gifts "blood money"?

The morning after his birthday party, Amir opened his presents joylessly. To him, each gift was tainted with Hassan's shed blood. He knew Baba never would have thrown him such an extravagant party if he had not won the tournament, and to him the victory was inseparable from Hassan's rape. Baba himself gave Amir a coveted Stingray bicycle and a fancy wristwatch, but they too felt like "blood money."