I have no doubt that this is part of the thinking. The animals are victims of the system, whereas the prisoners are conscious actors who deserve what they get. The animals are being stored until adoption, while the prisoners are being punished.

A lot people think prisons should be made as inhumane as possible as a deterrence to crime (or because they view criminals as inherently a different species of being, outside the human community). Humane countries such as Denmark and Netherlands have very humane treatment for their small number of prisoners.