at the checkpoint, hows amir's cowardice a source of embarrasement for baba?

At a checkpoint, the Afghani soldiers would have let the truck pass without issue, but one Russian soldier demanded a half hour with one of the refugees, a married woman. To Amir's dismay, Baba defended the woman, telling the Russian soldier that he had no shame and that he would "take a thousand bullets before [he] let this indecency take place." Amir felt ashamed that while Baba would give his life to save someone, he did nothing to save Hassan.