"Why believe in a god? Just be good for goodness' sake."
Why the heck should I?
That's precisely the question atheists struggle to answer. Sure, atheists can recognize moral truths and act according to them. In short, they can be good without God.
But as Melinda Penner points out, the question isn't about being good; it's about explaining good, and evil:
It's about an explanation for how these categories of universal, immaterial properties fit in a natural, physical universe if God doesn't exist.Note: For an extended treatment of the grounding problem for atheists, see Paul Copan Can Michael Martin be a Moral Realist?
The fact that atheists can be good isn't challenged by theists. And it actually makes sense in a Biblical worldview since all human are moral creatures and capable of great good and great evil. The problem is that atheists can't explain the existence of these categories.
We can be good for goodness sake. Even theists believe there is an intrinsic value in being good, and goodness just motivated by fear of God. Virtue is a love of the good. God loves the good, I believe, and we should love what God loves.
The ad confuses the grounding question once again. It's easy to assert goodness. It's harder to explain it without God.