There is a political agenda involved. DuVernay said as much when she told Rolling Stone magazine that the original screenplay for Selma was changed because, “I wasn’t interested in making a white-savior movie.”One of the dark secrets of Hollywood is that the shooting scripts of many movies are worse than the original scripts, thanks to a director or producer with more power than the writer. I've read the original script for Saving Private Ryan; so far as I remember, every one of Spielberg's changes weakened it. I haven't read the original script for Selma, so I can only speculate based on DuVernay's comment.
But this is my best guess: The story of the civil rights movement is a story of people of all races uniting to end the white supremacy promoted by the South's ruling class. But in that story, people like Johnson are important parts of ending segregation. DuVernay has classic Hollywood instincts that I first heard expressed in another movie, The Man Who Shot Liberty Valance: "When the legend becomes fact, print the legend." While claiming to be interested in the people, DuVernay wanted to make a black-savior movie instead of a movie about many human saviors.
Mind you, I still want to see Selma.