The first question for such a person is how they define "prison". Chances are it's narrower than you think.
Increasingly, criminologists have been developing approaches to restorative justice - concepts that offenders should be 'making amends', not simply bearing a punishment, and that a major objective of criminal justice is rehabilitation and reintegration.
In that light, the American incarceration-heavy approach has at least two problems: They incarcerate too many people, and in conditions that are not conducive to reintegration.
There are few who would argue against institutionalization of dangerous criminals, but a style of incarceration modeled after medieval dungeons isn't really a great way of reintegrating people.