Defining atheism as immorality is absurd, and is obviously a label created by the religious, the old crusade-speaking "you're either with us or against us" mentality. Because religion tends to be very complicated (with issues, interpretation, etc...), some people think that atheism must be complicated as well.
Here is how I see atheism: Atheism is not complicated at all. In fact it is very simple, so simple that it's difficult for some to grasp the concept. It is the belief that there is no god. That's it, nothing more. It is not what atheists "are", it is what they are "not" - they are not theists. It is not a religion. It is not an attack on religion. There are no teachings. There is nothing attached. Morality has nothing to do with atheism (or theism). Characteristics of an atheist are not a result of what a religion labels them - atheists wear their actions, not labels. Actions speak louder than labels. Atheists are free to be judged and tested by their actions. The only common thing among all atheists is that they do not belive in a god. (There are different degrees of atheism... strong/weak/agnostic atheism but that is splitting hairs) What were humans before god was discovered/invented by man? Atheists. Did that make them inherently immoral or less human?
Here's an analogy: Religion is like a board game, except each player has a different rule book or interpretation of the rules. They look over and see someone (the atheist) doing something else unrelated, not playing the game, carrying on with life. The players accuse the atheist of cheating, when he's not even playing. They say he's losing the game, when he's not even playing. (Ironically the players each have a "win card" that they can use to win the game at the end, regardless of how many "points" they each scored during the game.) Meanwhile the atheist is living life, free of the complications of theism, wearing and owning his actions on his sleeve instead of covering them up with a label.