Simply put, Dietrich Bonhoeffer was appalled by the racism in America. Even some of the pastors of the church, the spiritual shepherds, were racist! Bonhoeffer wrote:

The way the southerners talk about the negroes is simply repugnant, and in this regard the pastors are no better than the other!

(Bonhoeffer, Pastor, Martyr, Prophet, Spy by Eric Metaxas, 114)