In the earliest parts of the Reconstruction Era, the Republicans found the black codes to be very objectionable because they seemed to indicate that the South had not learned its lesson from the Civil War.
Early in the Reconstruction Era, many state governments in the South instituted black codes. These laws were meant to ensure that African Americans would remain available for use as laborers and to ensure that white supremacy would continue. This annoyed many Republicans from the North. Some of them truly believed in rights for African Americans, but many of them were simply angry because the South was not acting as if it realized that it had been defeated. It appeared to them that the South was trying to remake the system it had had before the war. The black codes seemed to some to be an indication that the South was going to try to go on much as it had before the war, not acknowledging that it had lost.
Thus, the Republicans’ desire to have the South admit it had lost and act as if it had learned a lesson led many Republicans to object strongly to the black codes.