Do you think the purpose of prison is primarily to rehabilitate prisoners into society or to give just, deserved punishment for offenses to society? If you agree with the former, then making conditions better than the legal baseline could humanize the prison population and increase their chances and motivation to reassimilate themselves into society. If you agree with the latter, then I don't see a good argument for bettering the conditions of prisoners. From a basic utilitarian standpoint, it seems very clear that policy guided by the the rehabilitation principle will result in better outcomes than policy guided by the punishment principles.