It is a misclassification to identity a group of individuals with a like-minded belief/value structure as a religion.
They could be a nation, a subculture, an advocacy group, etc.
Atheism is just the absence in the belief of a supreme being(s). Most Buddhists, by technical definition, are atheists along with some other religions. What separates atheism is the lack of agreement beyond that tenet. They don't meet together and discuss their disbelief, there are no special ceremonies to celebrate the entrance of new atheists...It just exists.
I can see atheists' argument logically though. I think embittered active atheists exist more as a result of these individuals viewing religions as tools of control rather than beneficial contributors to society. To them, the concept of god(s) is a lie, one of the greatest lies in human history, and they might feel the need to compel others to see "truth".
Religion is incredibly powerful and manipulative on a society. Consider in the past how civilizations used polytheistic deities to consolidate power, justify authority and wars, and establish a basis for common law.
I mean the concept of god(s) has evolved so much over time, and is used to explain reality and establish normative behavior for societies.
I still think religion will evolve even further, civilizations went from ancestral spirits, polytheism, to monotheism. It will likely change even further over time.
A good example of how influential religion is...most individuals practice the same religion as their parents. I would argue that if an American child that was born from two practicing Christians was plucked from birth, and raised in the Middle East they would be Muslim. Today children that are adopted from China are brought over, and usually raised as Christians and grow up as practicing Christians. However, if they were adopted in India they would likely be Hindi.
I am surprised most individuals do not think about those types of scenarios. Most individuals blindly accept the faith they were raised in, and rarely deviate from that path.
I thought long and hard about the religion I was raised in and after consideration felt that agnosticism was a good route. I have examined Buddhism a bit since it seems very...logical.