It's not being used as a proportionate punishment. Many incarcerated people were never even proven to have committed a crime, and in some cases never even faced trial. There's also no small number of people who were imprisoned for a crime someone else committed thanks to misbehavior on the part of prosecutors, police, or dishonest "witnesses". Sometimes those people even get executed.

The murderer working 8 hours a day in a prison is an outlier. A huge percentage of the prison population are normal people like you or me put in a bad environment being abused inhumanely because people have decided that they deserve it for some reason.