Altruists must regard punishment as inherently immoral. And they do. Leftists are altruists. That is why they try to pretend that penal institutions do not inflict punishment. The punishment motive is the flip side of the profit motive.
Altruists oppose both.
Therefore, if punishment cannot be entirely abolished, it should be watered down to impotence. Now we can understand the actual meaning of the mantra that punishment does not deter. That mantra, which seems to be merely a false statement of fact, is actually a moral principle, derived from the premise of altruism. It really means that punishment should not deter, that it must not be allowed to deter. We can also understand the leftists' peculiar combination of slobbering sympathy for some, and stony indifference to others.