Not capitalizing the word "God"--question

Did writing "God" and not capitalizing it start as an atheist jab against Christians, or is it just laziness on the part of the writer? I'm seeing it a lot lately.

Hate to be pedantic, but if the person writing it is referring to a general concept of gods, rather than the monotheistic God, then he/she should write "I do not believe in 'a' god," not "I do not believe in god."

If the word is not being capitalized because the writer believes God is imaginary, then this is arbitrary because the same would hold true for other fictional characters such as "bugs bunny" or "speeed racer."