That's a generalization. Many of us atheist define god by abrahamic terms. Some of us do not know other gods to consider them part of the definition of how we define (or use to defining as god). So, it's a default for not knowing how other people see the same word.
That, and it is confusing when a believer says they believe in god (to flip the switch). We ask, well, which god? If you say that, without having an already set definition of what we think god is, you could believe in two different gods that contradict each other.
So, it is ironic on both sides. Though, I wouldn't generalize. The word God I am used to hearing it as an entity in a Abrahamic concept. Other "gods" I would not call god since it doesn't fit in what I'm used to defining him. For example, Santeros don't use the word god, they use Olafin (Which I know I spelled wrong); so, I can't be an atheist to their deity since I don't know how they define it. Once they say "god" then my Abrahamic cap comes on. It's not the fault of the atheist but his or her upbringing (or for me, environment not upbringing) that defines the words we use.
So, I'd give us a break. Believers can no more define their god in a concrete way than us atheists. And to think how an atheist can define god is beyond me. The term doesn't make sense to me. Some of us use it for a language convenience. Nothing more.