Legislating morality

I often hear "you can't legislate morality" but that doesn't make sense to me. Murder is morally wrong and goverment has laws against murder. Maybe some people mean that laws can't change the morals of the people -- and I agree with that. But laws can change the actions of the people to reduce behaviour that is "immoral". Laws against murder don't prevent all murder but they do reduce it signifcantly. "Moral" laws help prevent people from harming each other.

The question then becomes: Which morals should be made into laws? I believe that answer to that varies by level and location of the government jurisdiction. i.e. Murder is generally a state crime and pet laws (leash, doo doo pickup, etc) are generally local crimes.