Why is Atticus sitting in the jailhouse in To Kill a Mockingbird?

Atticus sits on a chair outside the jailhouse in order to make Tom Robinson feel more secure and safe.  Atticus has an air of leisure...reading the paper, but obviously expecting visitors.

A group of men come to the jail and explain to Atticus that they have come for Tom.  Unbeknownst to Atticus, Scout, Jem, and Dill are hiding in the woods watching the entire scene.  Scout, scared for Atticus' safety, runs to his side.  She realizes too late that she has come among men she does not know.  However, when she recognizes Mr. Cunningham and speaks of the goodness in his boy, Walter, and how Atticus has helped the Cunninghams in the past, she unwittingly changes Mr. Cunningham's mind about the way things should go that night. He instructs the mob to leave.

So, to make a long story short, Atticus sits out in front of the jailhouse for two reasons:  one, to comfort Tom Robinson in his hour of need, and two, because it is the right thing to do.  It is not a black or white thing--it is the simply the right thing regardless of race, religion, gender, sexual orientation, blah, blah, blah.  Tom Robinson is not just a black man.  He is a human man who deserves a fair trial...not a midnight lynching.