Of the three men in the diner, Sam is the least disturbed by the encounter with the killers and the fact that Andreson will very soon be fighting in that big heavyweight ring in the sky. Fittingly, he is physically removed for much of the action as well. He’s out of sight in the kitchen for the first big chunk of dialogue, and he’s brought out for only a moment before he’s sent back out of the reader’s view again. When Nick returns from the boarding house, Sam pokes his head out just long enough to declare that he’s not even listening before shutting the door. (The 1927 adult male version of, "I’m ignoring you! Do you see? Do you see that I’m ignoring you?!") Sam is at the extreme opposite from Nick in terms of getting involved. He doesn’t want to be involved, he doesn’t even want to know, and he doesn’t think anyone else should, either.
But before we write Sam off as callous, we have to think about the fact that Sam is black in 1927. That he is repeatedly called "the nigger" is a clear indication of how society views him. Based on what we guess about his life experiences, it’s not too surprising that Sam can’t bring himself to be as proactive as the other characters. This goes a long way in explaining his propensity to just stay out of it – not because he doesn’t care, but because it’s been proven to him that you just can’t change certain things about society – even if those things are illogical and downright awful (like racism).