Remember, anti-theism is largely a reaction to fundamentalist religion. Many of us atheists were very quiet about our atheism years ago. Then, through a variety of ways, we recognized that there are many religious fundamentalists who are very socially and politically active who wish to install their version of a theocracy upon all of us as best they can. Because of this, we've recognized that we have to get organized to prevent this from happening and to be able to maintain the very existence of our naturalistic worldview, and not be discriminated against. Paramount to this of course is the principle of secularism: the separation between religion and government. The wall of separation must continually be defended from theocrats and the like. So, it would be nice to be able to just be an atheist, or a naturalist, but to be an atheist activist, it usually takes some degree of anti-theism. It's interesting to note however, that promoting secularism doesn't technically require an antipathy towards religion, just its encroachment on government. But the two are often hand in hand. An anti-theist is against religious belief. It is the belief that gives theocrats their motivation.* That's one salient reason why I'm against religious belief. I certainly want all religious believers to modernize, be moderate, and be pretty damn liberal in their theological views. But it's hard for me to picture myself advocating for liberal religion, instead of no religion, because, although I find liberal religion more tolerable, I still find it hard to believe. Nonetheless, encouraging the moderation of religion, seems to be imperative.
* And yes, of course, religion is not the only thing that motivates people to power and abuse, plenty of other non-religious things do too. I shouldn't even have to write this, but feel I do.