Does it make any sense to ask what is the case for atheism?

[Angrboda]

I think you need to look beyond the surface at what the point of the justification being done is. In this case, the justification is justification for rejection of a theist claim, or for rejection of a piece of evidence or argument for the theist claim. Again, the substance is a negative, rejection of theism; not a positive, an argument for non-belief. If there were no theist claim (eyewitness testimony), there would be no need to provide a reason for its rejection. You don't need to provide justification for rejecting a position that nobody holds. You do provide reasons for rejecting positions people do hold. Sam Harris has said that atheism is just the noise people make in the presence of unrealistic theistic claims. I'm not sure I wholly agree with Sam, but in this case his analogy is sound; nobody felt any need to justify until theist claims to truth were introduced.

(ETA: The point might be made clearer if I were to suggest that I had a 12th century eyewitness account of abduction by aliens. Rejecting such an account as evidence for the existence of aliens is not making a positive case for a-alienism, but a negative case against the claim.)