On theism, why do humans have moral duties even if there are objective moral values?

[brewer]

I was listening to William Lane Craig's moral argument. Even if we grant him that God exists and objective morality exists and god is the basis of objective morality, why do humans have moral duties? Why do we have to be good and do good other than for a selfish interest in going to heaven? He just claims that we do have moral duties (just like he claims by fiat that objective moral values exist). But why, on theism, do humans have a moral duty to be good?

Well this is easy  and it's a triple edged sword. Behave with moral  actions = heaven. Behave with amoral actions = hell. Behave with amoral actions yet repent = heaven.

It's kind of a weird Catch 22. Which ticket do you think the true christians believers want?

Don't get me started on catholics and their ability to purchase forgiveness.

And remember humans created god, heaven and hell. Think about it.