Are you suggesting that any person whose position is based only the lack of evidence that supports the existence of any God, and participates in religious forums, are NOT RATIONAL? There are some religious forums that do prohibit Atheist attending their forums, under the guise of protecting its members and their religious agenda. From what perspective are you deciding what is rational? Yours? Or, are you just trying to shame Atheists to leave?
Atheist contribute to religious forums for social, humanitarian, and personal reasons. They may want to provide many people with the answers to natural phenomena, that don't include, "God did it". They might want to provide encouragement for early indoctrinated minds, to begin the journey of regaining their lost innocents, creativity, and their natural curiosity. Maybe they just feel that if 2 1/2 Billion people think that the king IS wearing clothes, then maybe they just need special glasses to see it. Maybe they're just hoping to find just one proven miracle, one supernatural event, or just one piece of evidence that a God truly does exist. Once evidence is produced, their position will disappear. Maybe they just want to confront all the mischaracterizations, lies, and misrepresentations of all Atheists and Atheism. The kind that many believers enjoy depositing. Atheist will usually restrict themselves to non-religious topics, science related topics, or topics in which Atheism is the subject. Very few are involved in Biblical interpretations, sermonizing, Comparative Religions, or Biblical storytelling. Therefore there are hundreds of other threads you can attend, without any Atheists involved.
For some atheist there maybe some ego gratification, knowing that proof of the existence of a God is unknowable and unfalsifiable. But then the converse is also true among believers. There may be ego gratification knowing that proof of the NON-existence of a God is also unknowable and unfalsifiable as well. For me, I do not live in a world of NON-existence. I live in a world where things exist or they don't.