Here is that version of the problem of evil again:
(1) Gratuitous evils probably exist.
(2) Gratuitous evils are incompatible with the God of theism (omnipotent, omniscient, all-good).
(3) Therefore, the God of theism probably does not exist.
Now it seems on its face that only atheists who are moral realists can use this argument. Without believing that there are some truths about what is right and wrong, and therefore some non-relative truths about what act a good being should be expected not to do, premise 2 cannot be affirmed. But even one of the naturalist commentators on this blog accepted the conclusion that if naturalism is true, then a non-realist ethical philosophy follows. So if it can be argued that naturalism entails non-realism about ethics, then it follows that naturalists can't use the argument from evil against theism, because they cannot affirm (2).