The subtitle of this excellent story, to my mind, should draw our attention to the way in which walls are a prevalent symbol in the story. Consider how the setting of the narrator's offices are described. Not only are they located on Wall Street, but they also have a view of nothing more than the walls of other buildings:
At one end they looked upon the white wall of the interior of a spacious skylight shaft, penetrating the building from top to bottom... In [the other] direction my windows commanded an unobstructed view of a lofty brick wall...
The high number of walls and their significance makes us think of the theme of freedom and the way in which the freedom of Bartleby becomes ever more restricted as the story takes its course. Note the way in which Bartleby spends hours staring at the wall, so much so that the narrator describes him as being lost in a "dead-wall reverie." Bartleby is a character who seems to lack freedom in his life, and the high number of walls only serves to reinforce the way that he is imprisoned by life.
The story thus seems to be a meditation on the nature of human freedom. Is Bartleby the victim of the capitalist system that we have today that forces people to exchange their labour for paltry wages? Or is his lack of freedom more philosophically based as he struggles to grasp the significance of his life and debates the meaning of his existence? Either view is possible, but the subtitle of this story, giving us yet another reminder of the symbol of walls and how they represent imprisonment, asks us profound questions concerning our state of freedom and/or imprisonment in the world and the extent to which humans can ever be truly free.