A frequent commenter wrote, in our thread regarding crime and leniency in Great Britain:
Based on these trends, 2% of the adult population in this country is likely to continuously break the law. Punishment seems to work dandy, yessirree!
The commenter's sarcasm is absurdly misplaced. Holding those people in prison definitely prevents them from committing crimes against the public during the period of their incarceration. In this way, incarceration most certainly works to reduce crime.
It is true that a punishment should fit the crime, but that means not only that it should not be too severe but also that it should not be too mild. The appropriate severity of punishment for any particular crime is a valid matter for debate. What is not debatable, however, is that incarceration reduces crime. It does so, at the very least by taking the criminal out of circulation.
It is only when one shifts one's concern exclusively to the welfare of the criminals over that of their real and potential victims that one can see inarceration as risible.