Some forget that religion implies a set of beliefs, a system, related to the cause, nature and purpose of the universe, and usually comes with one form or another of religious practices and observances. Militant atheism can become a religion or hold a place essentially the same as religion in some people's life, but most atheists I know don't believe in a cause or purpose, and let science be their guide as to nature. They don't make it a religion, no more than, as you say, people make lack of belief in Zeus or Santa Claus a religion. This isn't a religion, this is not leaving a place to religious beliefs and practices in your worldview.

Actually, your alternative way wouldn't anger me. I wouldn't mind being labelled agnostic. Lack of belief in a superior being or believing that even if there was such a thing religions from A to Z remain the pure invention of men, it's in the end pretty much the same to me - at least in the ways that matter.

As for my own alternative for how to anger an atheist, I'll pass as my proposals could serious piss off some religious people and it's not fights I ever cared much to get into.