is that they are putting on a show.
From the first moment of letting Newt Lee go because hanging a Black man wouldn't be sufficient for the crime, to the literal song and dance of the Factory Girls' testimony, to all of the "fun" numbers being the lies about Leo Frank or ignorance of his situation (and when telling the truth or calling for justice in his situation, the mood is killed and/or music stops), to immediately jumping up and celebrating after Leo Frank is convicted, the musical is absolutely clear that the whole "search for justice" is about putting on a show.
By having Ben sit on stage for intermission, the musical refuses to allow the audience to remove themselves. Ordinary intermission has the lights come up and people get up, go to the bathroom, talk, do their thing. They are no longer in the magic of the show. But not now. Now, a man has been sentenced to death and is right in front of you. And you can't get close a curtain on him and get 15 minutes of peace - you are forced to be part of the musical. You don't get to ignore the miscarriage of justice for 15 minutes and then resume the musical after a break. You have to live in it, just like he did.
You're forced to be part of the spectacle. You don't speak up and save him. You have three possible reactions: 1. You simply watch him wait for his death for a crime he didn't commit, 2. Go about your daily life ignoring the innocent man in front of you, 3. or, worst of all, you take advantage of the innocent man who's been wrongfully convicted (the people taking pictures at the show and/or enjoying the acting, taking pleasure out of a negative situation).