Why was the emancipation of a slave not stipulated for the expiation of ẓihār, and why was the penalty similar to that for a murderer?

The majority of scholars apply the restricted meaning to the unrestricted one regarding the expiation for Dhihar (declaratory divorce) and the expiation for an oath, thereby stipulating faith in the emancipated slave. As for why the punishment is similar to that of a murderer, Allah knows best, for "He is not questioned about what He does, but they will be questioned." The wisdom behind the exaggeration in the expiation is to deter a Muslim from Dhihar.