I think that this becomes one of the fundamental questions of the novel. Due to the fact that the American's purpose is unclear, I think that it is here where Hamid allows the reader to insert themselves. The American's purpose is very ambiguous and I tend to think that this is deliberate. Hamid makes the American a portal through which we, the reader, enter the narrative. The reader's own presuppositions and perceptions filter through which the American can be viewed. In this, Hamid has been able to construct what a paradox America, itself, in its relationship to fundamentalism and terrorism.
If the reader is likely to accept that the "war on terror" is a condition that defines the modern setting, then the American represents the force of the "avenging angel" sent to destroy "the enemy." In this case, this would be Changez. The American would thus represent that force which is meant to stamp out the threat of terror, wherever it may be. If the reader is more inclined to see a globalized world in which cooperation and mutual dependence is evident, than the American is there to simply understand Changez's point of view. He is a portal through which Changez's voice can finally be validated. He is not there in any military capacity, but rather is reflective of how the strength of the world's greatest democracy lies in the validation of voice.
It is in this function that the American operates. In presenting the American, and in a sense, the reader, in such a divergent manner, Hamid is making it clear that the notion of what "America" represents is an example of the greatest paradox in the modern setting: America as a source of voice validation or voice negation. It is in this where the American operates, representing the section of America where hope and promise lie or where looming imposition is too close for comfort.