You're right, it doesn't make sense. As atheism is defined as the lack of belief in a higher power, it is implicit that all things in existence are under the physical laws of universal dimensions. This means whatever occurs, is a logical outcome of empirical data.
From that you can infer that the actions of anything, including humans, is a predefined notion, no choice is made, it is only an illusion that choice is power
An atheist in ideology believes that morality is unnecessary, because there is no free will to discriminate what is moral or amoral, solely the context in which something happens is the impartial determinant of all things
In order for moral, you must be conscious of some power that you posses or is bestowed upon you that illuminates choice, which requires belief in a higher power.