Atheism is practiced in a proactive, law-changing manner and is therefore a belief system. Simple non-belief in gods does not have the power to be a cause. It would be a little more than a shrug. For people who spend lots of time writing articles, promoting legislation, and arguing for atheism, atheism is a cause.
People see atheists involved in various forms of political or social activism and conclude that atheism itself must be some sort of belief system and cause. This myth bizarrely confuses atheism with the causes, interests, and goals of people who happen to be atheists. If an atheist promotes church/state separation, that's evidence of atheism being a cause. If an atheist promotes sound science, that's evidence of atheism being a cause. None of this makes the least bit of sense.
Frankly, I find this to be one of the more inexplicable myths which people spread about atheism. If the mere fact that a person is engaged politically means that some quality of theirs becomes a "belief system" and a "cause," where does this end? Is being male a cause? Is having long hair a belief system? Of course not, that's sheer nonsense; yet it's effectively the same as saying that atheism is a cause or belief system simply because an atheist is politically or socially active.
The first and perhaps defining mistake this myth makes is to describe atheism as something that is practiced. If it were, then it might be conceivable that atheism would be more than mere disbelief in gods. In truth, however, atheism is not something practiced nor is it something one improves, cultivates, grooms, trains in, or rehearses. Atheism is a quality, like being apolitical or having an astigmatism. Atheism is not "practiced" in any manner, not even in a "proactive, law changing manner."
Atheists are certainly engaged in proactive, law-changing causes, but that's not "practicing" atheism. That's simply being an engaged, active citizen who happens to be an atheist. By the same token, a theist who is active in some political organization is not thereby necessarily "practicing" their theism. They may be engaged because their religion teaches or promotes something, but that's a far cry from practicing their theism. Theism is simply the presence of a belief in some god, and it's not something that one practices.
It's noteworthy, and I think very important, that theists who repeat this myth only do so in the context of atheist activism that they oppose. Atheism is described as a belief system and cause when atheists promote church/state separation which the theist disagrees with, or when atheists promote sound science against an anti-science religious ideology that theist defends. These same theists don't try to argue that atheism is a belief system or cause when atheists support gun rights or gun control, stronger penalties for murder, or increasing the minimum wage.
Why do some theists employ this double standard? I believe it's a sign that a person is so preoccupied with themselves and their own interests that they are no longer willing to stop and listen to others. For such theists, opposition to church/state separation or sound science are intricately bound up with their theistic religion. If an atheist disagrees with and opposes them, it's assumed that it's because their positions are also bound up with some religion a religion identified as atheism all by itself. Similarly, other theists who disagree with and oppose them are dismissed as not being "real" Christians or "real" believers in God.
What we have here, then, might be described as a lack of imagination: the religious theists in question assume that everyone is basically like themselves, organizing their lives and beliefs in basically a similar manner and acting with basically similar attitudes. Atheists must be in denial when they insist that they can adopt particular political and social positions without that being part of a "religion" called atheism. Theists must be in denial about their atheism when they insist that they can oppose other theists.
One thing which the myth gets right is that mere atheism, all by itself, need not be anything more than a shrug there is nothing about atheism alone which leads a person to any sort of activism. This is why people aren't active with any political or social causes merely because they are atheists. This is not to say that their activism is unrelated to their atheism they will hardly engage in activity to promote theocracy, after all but their activism does not and cannot follow inevitably from their atheism.
Whatever they are involved in also has theists involved, often for much the same or at least similar reasons. There will also be atheists involved in opposing political and social causes, as well as plenty of atheists who don't have a position on the matter. It's no coincidence that the exact same thing is true of theists, because both atheism and theism are compatible with a wide variety of philosophical, political, and social belief systems. Neither atheism nor theism forces any particular political, social, or even religious conclusions.