The lawyer motif
This novel has a motif about lawyers, which seem to represent exactly what they represent in real American life, which is the ability to earn or lose money in American courts which settle disputes. Now, the homeless man is not a lawyer, though the title calls him The Street Lawyer, so this book is about justice. The lawyers provide a backdrop against which the homeless man's behavior can be contrasted.
The eviction
The wrongful eviction is done because the lawyers know that the poor man cannot afford to fight them in court, so they can just do whatever they want, because that man is not even enfranchised enough to fight them. The eviction represents the absolute injustice of corporate greed, and it represents a kind of true victimhood.
The gunpoint hostage moment
One might argue that the only real reason guns are in the story is because Grisham writes thrilling novels, and this is a common motif in his genre, but the gun does serve as a symbol nevertheless. The gun symbolizes the universal standard of human behavior, which is human death. By threatening them with death, he is showing that they have been wrong to view him as inferior; with a gun in the room, he subjects them to his own wrath. That positions the reader in a dilemma about gun violence and heroism. This is ethically dubious.
The symbolism of homelessness
The gunman is also just as frustrating a character as the lawyers. He treats homelessness as an absolute humiliation, but doesn't that imply that he thinks homeless people are beneath him? The symbol of the homeless is one that the novel adopts about this man, but there are literally millions of homeless people—many in every human city just about. The dilemma of homelessness refers the reader to consider their own assumptions about homelessness.
The admission of guilt
In perhaps the most uneventful climax of any Grisham novel ever, the company admits they were wrong. This is a blatant symbol that the company is wrong. Wrong about what? Wrong about killing the homeless man with a sniper rifle. In case the symbolism doesn't speak for itself, Grisham is saying, "The company is wrong for mistreating the homeless."
