[N.B. This reply also includes a response to your other post that you've now deleted.]
And yes, they don't have to explain. They already have justified their reasons.
At bottom, disbelief in God has no burden of proof because the object is not an innate idea and nor is it apparent in general experience; and this uncontroversial point is simply explained as a lack of evidence. Now there is a difference of course when in response to There is no God the theist presents an argument for what is believed to be evidence for the existence of God, e.g. the cosmological argument or something of that ilk, and then in that case the disbeliever must step up and support his/her assertion; but even if arguments to the existent of God are refuted by the disbeliever, as they can be, it still doesnt prove non-existence. So in the case of an existential proposition such as this the burden of proof is always with the person making the assertion because the doubter cannot prove the non-existence of the supposed entity. In simple terms, then, its for the theist to deliver the goods.
All assertions have a burden of proof, of course they do, but the thing in question, the central existential premise: God exists isnt safe on account of no convincing objection being made. The objectors lighter burden is simply the requirement to respond with argument to the theists claim, while unable to disprove it since that is a logically impossible task; and contrarily the theists necessary burden is not merely to rebut the objectors arguments but to demonstrate the truth of the central premise:
Theist: W (God exists)
Objector: Not-W because of X
Theist: X is false because Y
Therefore W
Note that this hasnt demonstrated God exists (W), it has only demonstrated that the objectors argument fails. The burden for the positive case remains, however.
Disbelief is not represented by, "Why don't you believe in God?" but by, "I don't believe in God."
Disbelief is not represented by the question Why don't you believe in God?' Ive no idea where youve got that from; it was merely a single working example and certainly not something Ive stated as representing disbelief! And nor is disbelief represented by an assertion! Disbelief is held through a lack of common evidence as Ive already indicated, and Gods non-existence does not follow as a necessary assertion and a consequent of disbelief. One can disbelieve in God, as many do by simply observing what there isnt, but negative assertion is treated differently as described up the page.