Many theologians and philosophers actually do argue that without God, we lack an objective ground for morality. Of course, they do not make the claim that non-believers are amoral. The problem is indeed a problem of grounding (ontology). People may come to know what's good and bad, and what they ought or ought not do. But when people make claims such as, "This is wrong," we want to know what they're referring to.
Your essay inspires my next essay. I will attempt to offer a thesis on moral realism without an appeal to a deity. I hope to catch you there. :)