I have to disagree with the title. Many atheists I've met are more religiously educated than theists.
I was having a discussion with a bunch of cultural Christians the other day, and they found it amusing that I was the only one who'd read the Bible.
Much as it makes me chuckle too, it makes sense that people who have made a more firm decision about God (whether it's of belief in a specific dogma, or disbelief entirely) have invested more effort into things than those who are somewhat 'lukewarm'.
As/if atheism becomes more normal than cultural Christianity, then I suspect it would be more common for atheists to have no real grounding or education in religion of any type. Not like it's a requirement. But certainly in the environment I grew up, a somewhat loose form of Christianity was assumed and normal, whilst deviation from that required some level of thought, effort or decision.