Know thy enemy...
From their writing and use of the word atheist, it seems that they choose the word with the intention of implying that certain scientific theories were developed under the belief that no god exists. "General Theory of Evolution...was derived from atheistic presupposition." I'd imagine leaving god out of a theory would make it "atheistic" in the sense that, well, the theory isn't theistic (Atheism is derived from the Greek word "atheos" - "a" meaning no or without and "theos" meaning God). The general theory of relativity would also be an atheistic theory in that sense.
Clearly the reality is not that scientists said "we made this theory on the belief that there is no god," but instead "we made this theory based on observations without considering if there's a god or not." The creationist sites word things, with clear understanding of the connotation behind the word "atheistic," in order to use the negativity around the word to their advantage.
I think the overall feeling I have gathered from reading this site is that they believe if you don't say "god did it" then you are saying "god didn't do it." This is why they want creationism taught in schools, they feel that without saying god was involved, they think their children will really be learning god wasn't involved.
I can't say I disagree; the more people learn about evolution and science, the less they believe that god is involved. I don't think that atheism reigns supreme in the scientific community for any other reason that people who once were deists aren't seeing any evidence for god, and yet are seeing evidence that clearly contradicts what they did learn about god as children. The site even acknowledges "the ability of the teaching of evolutionary biology to turn people away from a belief in god."
Funny how the truth and knowledge does that, isn't it?