This short story is told from a first-person point of view, as the narrator speaks only from his own experience. In this case, this point of view keeps the reader firmly inside the narrator's thoughts and closely connected to his feelings. What we know of the other characters, such as the 7-11 clerk and the ex-girlfriend, comes only through the narrator's own assumptions and observations. We assume the clerk is initially afraid because the narrator thinks that he is. It is possible that the clerk thinks otherwise, but by limiting the storytelling to just the narrator's perspective, we can understand his personal experience better.
This gets to the advantage that first-person limited point of view has. It is more intimate than other forms. It humanizes the narrator and allows the reader to better empathize with the storyteller.
This point of view also allows the narrator to more effectively do something that he does throughout this story; he wonders what other people think of him, whether it's the 7-11 clerk, the BIA kid, the ex-girlfriend, or the people on the phone at work. As readers, we are also meant to wonder if the narrator's assumptions of other people's assumptions of him are true or not.