It's a good question, though people tend to treat it as a thought-terminating cliché rather than exploring the implications. Why should murderers be punished, actually? Enacting punishment is an external incentive, a stimulus, supposedly structured to make the cost to the potential murderer higher than the benefit they hope to get by killing. Belief in punishment, therefore, is consistent with the non-free will position. But if there's no free will, then why not instead try to "solve" murder, and not have murderers anymore, by discovering the root causes that drive people to murder, and mitigating them? We'd all be better off!

On the other hand, free will implies that the mechanism of punishment may or may not be punishing to the murderer. We don't know what they feel in response to stimulus; they have free will! Like in the story of Br'er Rabbit, trying to determine a foolproof method of punishment that's hateful to the murderer is an exercise in futility, since we can't know their mind.