Hi Kjotleik,
atheism and agnosticism address different claims. Atheism addresses belief, agnosticism addresses knowledge. They are not mutually exclusive, one could for instance be an agnostic atheist, which, in my experience, most atheists are, meaning that one doesn't believe a god exists, but cannot know that as certain knowledge.
Imagine for a moment we are talking about unicorns. If I tell you that unicorns exist, and you don't believe my claim, you are an "a-unicornist". You have not implied that you are sure that unicorns don't exist and remain open to whatever evidence may one day be put on the table, but for the time being, you remain without belief in unicorns.
Likewise, the person who makes the claim that a god exists, has the burden of proof to present evidence to support his claim. People who are not convinced by his claim don't adhere to its theistic framework and are therefore a-theistic, without theism.