Why are atheist compelled to answer religious or spritual questions?

I think an issue with this question is that there can never be a uniform answer.

Atheists posit an all-inclusive negative in regards to an actual universal reason to do anything at all; that is, they actively deny the existence of anything which could dictate such a thing (and I say 'actively' because I do not suppose that every thought an atheist has is preceded or followed by the thought "God doesn't exist"). As such, the motivation and goal of an individual atheist's actions are whatever they happen to like at any given moment, which means there cannot be a generalized answer to your question.

Given, most of the atheists I've debated with say that they argue because they have to deal with the problematic 'falsehoods' of Theism on a daily basis, and that they'd just like to do they're part on putting a stop to it. However, taken to it's conclusion, their motivation is still always reduced to "because I personally happen to like or dislike this or that", else they unwittingly denounce their precious negative and start talking like something truly is good and worthwhile.

I'll say this because I can prove it: atheism is anarchic by nature--it brings nothing to the table in regards to how and why people ought to do or think anything at all. The funny part is that atheists generally destroy their own credentials by merely behaving humanly. As Lewis said, "At the sight of injustice they throw all their Naturalism to the winds and speak like men and like men of genius. They know far better than they think they know."

I might want to change my use of the word 'anarchic' to 'nihilistic', but I hope you get my meaning.