It dawned on me a little after Obama's speech last Tuesday ...
The terms used to refer to those who don't believe in god, particularly God-with-a-capital-G, are all words that define the person, or people, in question in opposition to something they don't believe in.
Not that there aren't positive words out there that don't purely define one in opposition to something that doesn't exist to them, they just tend not to get used, or only get used in specific contexts for instance:
Maybe I was just thinking about it because, at least according to Obama's speech, beyond most (but not all) the major world religions there were the non-believers. Yet, not only are there many other religions, but there are also many spiritual people who don't "have religion," as it were.
That and too much religious rhetoric in a political speech from the leaders of a country that claims to have separated church and state begins to creep me out. The reason it creeps me out is that it starts the dialog from a position of exclusion. Even in acknowledging other belief systems, in order to be faith, by definition, it must start from the premise that those other belief systems are misguided or wrong (but we will treat them as equals until they finally come around).