The main problem here is that you are misinterpreting atheism. A rational atheist is not someone who categorically states with absolute certainty that gods and deities do not exist; rather, a rational atheist is someone who categorically states that he or she lacks belief in any kind of god or deity. The reason for this is simple: there has never been any event, experience, phenomenon or set of data/intelligence that has been interpreted by this person as being real and true ONLY due to the existence of some god or deity. The theist is merely the reverse, i.e., they have interpreted some event, experience or phenomenon in such a fashion. Since this is the case, atheism is NOT a belief system...it has no functional set of beliefs, dogma or ideology. It is nothing more than a rational statement as to the reasoning behind a LACK of beliefs. The only thing that separates the atheist from the agnostic is that the agnostic is, generally speaking, given to be more contemplative about matters of pertaining to the religious and spiritual. However, the atheist is, again generally speaking, content to live with this lack of belief.
What you might have been exposed to is something called "hard atheism". It is a type atheism that does try and assert with absolute certainty that there is no such thing as a god or deity. Since such a position requires absolute knowledge-and no one has that-it's statement becomes untenable. Such atheists are in the extreme minority and the overwhelming majority of atheists are those who espouse their atheism as I related above.
As to why atheists are here...why shouldn't they be? Religion has been a defining force in humanity's development for thousands of years. One need not be religious to discuss religion's past, present and future impact on humanity and as to what extent that impact should occur.
By the way, in looking at the three answers you gave for "will I go to heaven when I die?", you are incorrect...yes and no are perfectly correct answers to give provided those answers are predicated by your own personal beliefs (theistic) or your own acknowledged lack of belief in such a thing (atheistic). The answer "I don't know" is predicated by the lack in either acknowledging or making a personal decision one way or the other. Remember, you can't just be an 'agnostic' forever...but you can be an agnostic-atheist or an agnostic-theist.