SATCHMO

The problem is that atheism is just as much of a religion as any spiritually based world view. Rhetorically speaking, it is a religion that worships at the altar of science and reason, and uses these, in a materialist sense, to justify its own paradigm of thought, just as all religions use whatever resources avail them to justify their own paradigms.
Anywhere we find the act or mindset of justification there is a defensive posture.
Because a spiritually based religion is a faith based paradigm it relies less heavily on justification of its validity. Which is why you there is more of a defensive posture among atheists, and to a much lesser extent agnostics, than you do with adherents of faith based religions. The reason why this does not appear to be so is that zealotry of any kind is an attempt to justify itself through logic and reason rather than not justify itself at all through faith. It is one of those strange ironies that the most outspoken representatives of any world view, the zealots, are most often the ones who model for others their understanding of the respective religion or worldview, yet most often the model that is provided through zealotry brings forth the most distorted version of that model which they seek to represent.