It's mostly semantic but the usage of atheism as the absence of theism is quite new and not really used in philosophy in that manner. In the end it shouldn't really matter. It mostly matters for the religious that want to define you themselves instead of listening.
It matters a lot to atheists that don't want people to be confused. It's also a bit hard to be taken seriously when a small portion of your mob doesn't even understand what atheism is and then tries to tackle the bigger concept of deities with someone. Breeds the idea that atheists are idiots and don't understand what they're talking about.
