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. Both come from the premise of altruism. Being a loser qualifies one for altruist concern.
Altruism breeds sympathy for criminals.
Criminals, after all, are the world's ultimate losers. Every decent man loathes them. They must skulk in the shadows of life. They are heedless of their true interests. The life they have chosen guarantees their ruin even without legal punishment.