EDIT: I've had some independent critiques of this post, and they both made many good points. As such, I should look through this in detail and note everything that's totally invalid. As is, some random set of assertions that you don't agree with, and which undermine the argument, I've since reconsidered. EDIT ENDS.
Again and again, atheism is stated to be "just another religion" by people who, in the context of the first amendment, would rather it weren't. (Agnosticism doesn't get this nearly as much, possibly because, well, to me, it seems kind of wishy-washy.)
I invite those who make this claim to consider the vast history of things that we agree are religions, and take note: what are the religions doing, that atheism does not? Among other things, religions schism. Interpreting things very generously, we've got three members of the atheist spectrum: strong atheism (there are no gods), weak atheism (I don't think that there are gods), and agnosticism (we can't know whether there are gods).
(Why this last group doesn't apply methodological naturalism to the kind of belief espoused by, say, Pascal's Wager in the context of the vast multitude of available religions to convert to eludes me, but they probably have a good reason. It is quite likely that this good reason is that I'm a complete wacko for mixing philosophies like that.)
Any true religion would have schismed several times over by now, at the very least along political lines. If a liberal atheist and a conservative atheist have a discussion/argument/flamewar so intense that the internet itself is ripped asunder from its moorings and tumbles into the abyss, they don't call each other 'atheist lite' or 'atheist in name only'. They call each other passionless drones and money-grubbing nannies. Another possibility for schism, of course, is over doctrinal differences.
The observant reader will have noted that I outlined each of the three doctrines in a sentence each. Even if a religious doctrine were to start out that compact, normative influences would make it pick up the flavor of its community in perhaps as long a span of time as a generation. And yet, the core pure doctrine of atheism is preserved for centuries, without extraneous accretion. Wow! All the people crazy enough to use the KJV as the 'original' version of the bible should look into this 'atheism' thing! It seems, so far as uncorruptible teachings go, that we've got a lock! Ride your unicorn over here and we can talk.