It is immoral to steal, assault others, kidnap them, murder them, etc, is it not?
Think about it. Is murder immoral? Yes. Is that why it's illegal? No. It's illegal because you are generally thought to be violating someone's rights when you kill them.
You don't get thrown in jail for boosting the distributor out of my vehicle because it's some kind of moral failing. You get thrown in jail for boosting the distributor out of my vehicle because you are violating my property rights.
You don't get the law called on you when you burn a church, bail on your child support, or grab your co-worker's t**s because those aren't nice things to do. You get the law called on you because those are ways of violating other people's rights.
That said, I think morality is a good thing, and I think most, if not all laws aim to have a moral component in them.
But our nation wasn't founded because Our Founding Patriarchs held certain morals to be self-evident or inalienable. No, those guys were all about rights. And so we live in a country founded upon the idea of keeping our rights safe from the government, from our fellow citizens, and--most relevantly today--from organized self rightous religious....people.
So, although they did mention "God" occasionally, and "The Creator" and whatnot, they quite deliberately avoided writing Baby Jeebus or Leviticus or The Ten Commandments into our original documents, and they based the government which we enjoy to this day on the Enlightenment concept of Rights.