That only follows if you believe that free will implies moral responsibility, and that moral responsibility means punishment must occur, and that that means more punishment must occur. Why doesn't moral responsibility mean less punishment? What about the moral responsibility of those meting out punishment?

And in either case, both concepts are intangible and immeasurable, so using them as justification for something as consequential as imprisonment means something else much more tangible and measurable is being hidden behind those concepts.

I think it's just the exercise of power. That's why moral responsibility is only ever used to punish and never to stay punishment, because those wielding the argument aren't interested in those arguments being used to limit their power, only to exercise it.

The only thing that matters is effectiveness to reduce harm, and that is basically never spoken about by those in favour of incarceration.