Actually, I think that it can be better addressed by the study linguistic semantics.
Belief is a mental state, and, when you negate belief, there is an inherent linguistic ambiguity--whether the mental state falls within the scope of negation. So consider the ambiguous sentences:
"Henry does not believe that God exists."
Depending on context, it could be a denial that Henry holds the belief, or it could be an affirmation that Henry believes in the non-existence of God. In that case, Henry would hold a positive belief about whether or not gods (or God) existed.
Now consider words like "atheist" and "agnostic". Both are words that are inherently negative in meaning. They both define people who reject some kind of belief, but how could one apply the label properly to any individual? One can only do that if the object of the mental state--in this case, a belief--is known and understood. In other words, gods have to mean something to those who use the labels "atheist" and "agnostic". In normal conventional usage, we usually use the words in reference to people who reject the object of belief--gods (atheists) or ability to know of the existence of gods (agnostics). This is not to say that we can't extend the word meanings to a broader category--that is, the negation of the mental state itself. However, that would depart from the norms of usage. English speakers don't normally think of atheists and agnostics as people who merely lack the concept of gods.
The problem for atheists is that lack of evidence for the existence of god(s) is a motivation for rejecting belief in them, so they often feel a need to include that motivation as part of the definition of atheism. By insisting that the term only refers to absence of belief, they emphasize their position that atheism ought to be the default for everyone, often shoring up the opinion by pointing out that people who are religious still usually reject other religious belief systems by default. ("You and I are no different in whether we are skeptical about the existence of gods, except that you aren't skeptical about one particular god.")
Nevertheless, a lexicographer--someone who constructs word definitions for a living--should not be swayed by how people would like to define words. They should base their definitions on how people actually use words in most contexts, and that is an empirical question. I have not actually done a thorough investigation of the usage of words like "atheism" and "agnosticism", but my gut feeling is that people seldom use those words to refer to a mere absence of belief. Almost invariably, it is about rejecting the object of a belief.