This might just come off as silly but...
To many sects of Christianity spreading the word is actually a fundamental part of their faith.
Atheists don't believe in religion, Atheism is not a religion or even a belief (as many Atheists will strongly argue, even here). Therefore what compels an Atheist to place that message there other than as a jab to people who do actually believe something? It is not their obligation nor belief (disbelief?) that they should spread their 'good news' or 'faith' (lack of faith?) so why would they do such a thing? Maybe I just lack insight (disinsight?) but this whole thing seems pretty silly to me.
The real answer is that some Atheists are no different than Christians in more ways than they're willing to admit. They both hold their beliefs (disbeliefs?) as the absolute truth, and think its their right, nay, their duty to make everyone else belief it as well.
Plus it's always hilariously ironic when a religion thread turns into some atheists 'proving religious people wrong' whilst at the exact same moment other atheists complain about the jehovahs who push their beliefs on other people... ha
Oh and in case my point was a bit fuzzy, I think its ok for them to put whatever they want on the cannon, I just don't see why they would feel the need to form 'unreligious' groups. I mean I could form a 'we're not blind group' of people who can see, but whats the point? They seem to be taking atheism and making it into a religion... which its not supposed to be?