If they are "left to their own devices" and get a "slap on the hand" then why has the number of people in prison doubled in the last 20 years? Normalised by population there are 50% more people locked up than in 2000.
In the aggregate (rather than picking edge cases) New Zealand locks up a huge proportion of its population. How is that working out? There is no evidence that it works.
On the other hand, is there evidence that crime drops if we don't lock them up?