In the metaphor "a million crabs living in a million crevices," Amir is suggesting that people in America tend to create small, isolated spaces for themselves and prefer to remain within them, much like crabs in crevices. This implies a lack of interaction and connection with others outside their immediate circle. Therefore, the correct interpretation of this metaphor in the context of comparing people in America to those in India is:
They avoid interacting with people they don't know.
This suggests a more individualistic or isolated lifestyle in America compared to potentially more communal interactions in India.