Sith, I can think of 2 reasons why atheists get involved in religious threads. The first is that religious belief and custom permeate and pevert society to varying degrees. In some countries nobody is allowed to drink alcohol, regardless of their religious affiliation whilst in other countries it is not possible to have an abortion and in others shops are closed on a sunday. These are only a few examples but they illustrate my point. There are much deeper cultural effects that are taken for granted because we have always lived with them.
The second reason is that many atheists are concerned with intellectual honesty and understanding the universe as it is without looking at it through religious spectacles. As a result of this when believers post statements that are either fanciful nonsense or downright lies then atheists challenge them, because lies promote ignorance and confusion.
To give an example, Keyplus recently stated that humans are comprised of a body and a soul. Now nobody would dispute that humans have bodies but there is not and has never been any evidence that such a thing as a soul even exists let alone that humans have them. Keyplus stated this as a fact though it is only his belief. If it wasn't a downright lie it wasn't far off being one.