Why do atheists spell "God" with a little "g?"

I had a conversation today with a man we will call M, posting in the comment section for an Amazon review of our e-book, True Reason, which came up with the crocuses this spring.  (And whose cellulose leaves are due to drift down in physical form this fall.)  He admitted, in his initial post, that he was confused by a popular argument that God is the necessary source of morality.  Four rounds of back-and-forth showed that he really was confused, and seemed to want to draw others into that state as well.  But ironically, from all the confusion a measure of clarity seemed to emerge on a question I've often asked in recent years: why have so many Internet atheists taken up spelling "God" with a small "g" lately?