Atheism is largely misunderstood, as is the concept of agnosticism. What many people define as agnosticism actually falls under the category of atheism. This is what leads to the false idea that atheism is just another "religion" with it's own "beliefs", that being the non existence of god.

To explain, atheism is NOT a belief that there is no god, it is a logical assesment of available evidence. It does not claim the non-existence of god as an established fact, it works under the train of thought that there should be no credence given to the existence of god without actual evidence of such. Otherwise, you would have to be "agnostic" about the existence of literally any imaginary being or thing I can conjure up in my mind.
Agnosticism, on the other hand, purports that the existence or non-existence of god is still undecided and both are logically admissable systems of belief.

As for atheists who claim god does not exist as a fact, they clearly do not understand the proper logical or scientific method of interpretation of evidence. Even Richard Dawkins, the prominant and outspoken atheist, says that you cannot discount the existence of god.