The statement "I believe there is no $deity" is very different from "I do not believe in $deity", although it's easy to see why people mix them up. This is usually the point where people start bringing up leprechauns and pink unicorns. Theists often find this insulting, because let's face it, unicorns and leprechauns are silly. Comparing someone's deeply held belief in $deity to a mythical creature is usually quite offensive to the theist involved.
However, the point of those little arguments is not to demean religion, but to be thought experiments on the nature of belief and semantics.
Here's a little thought experiment that I think is a little less condescending. Please bear with me, as I'm making it up as I go along. Also, please get over your sophomoric giggles at the word “box”.
One's religious leanings can be thought of as a box. It's your box; make it out of whatever you like.
$deity is a fabulous treasure, hidden somewhere. Faith or belief is like a map to that treasure.
When you were born, your box was empty. You did not know that there was anything that could go in the box.
Your parents or guardians filled that box up with their own version of the treasure map, by believing themselves, and by exposing you to those beliefs from day one.
You probably don't remember when your box was empty, because back then, the rest of your brain was pretty empty too.
You know, because you've always known, that the treasure is real, and that your map will eventually lead you to it.
As far as you know, your box has always contained this map. It is a very basic part of your identity. Whether you are devout or lapsed, or simply 'spiritual' rather than religious, for as long as you can remember, your box has been full.
You trust your map, and you believe the treasure is real, for reasons known only to you. No matter where you go or what you do, you ask yourself "what does this have to do with the treasure?" The treasure is such a deep and intrinsic part of you, that you are unable to think of anything except as it relates to the treasure.
(This is of course an oversimplification. No insult is intended. But think of it fairly for a second. No matter what else is going on, you know that some day, your map will lead you to treasure.)
Someone who has never seen an 'empty box' can't help but wonder where the map went. Because all of your beliefs are based on the existence of the treasure, you can't help but see this empty box and missing map in that context.
You therefore assume, incorrectly, that an empty box really contains a map that shows the way to no treasure. You interpret the information in terms of your treasure.
What almost every argument I've read fails to grasp is that many atheists, if not most of them, don't have a map at all. Their box remains in its original state, empty. Not because they've never seen a map (most atheists of my acquaintance have studied religion in great detail), but because none of the maps they have seen were worth putting in the box.
Some people, the so-called 'Gnostic' or 'strong' atheists, insist that there is no treasure, and that all of the maps claiming to lead the way are in fact lies. Some believers even go so far as to empty their own box, having 'seen through the BS'. This is usually the position that both irritates and confuses theists.
Atheists have heard that other people think there's a great treasure out there somewhere. But they don't buy it. Everyone has his own idea as to what the treasure actually is. Every map is different, and none of them can agree on anything except very minor details. Atheists often find this silly.
The atheist position is usually something along the lines of “Treasure? What is treasure? Hey... you're the one who insists there's a treasure. I don't care one way or the other, but if you want me to buy your map, you're going to have to show me some proof that it's real."
But the only proof that people can provide are their maps, most of which have a little disclaimer that says “This map is true, because this map is true. Trust us.”
As yet, nobody's map has been any better than a crayon drawing on the back of a kid's meal place mat. "Follow these directions exactly and you'll find a great treasure. Don't follow these directions exactly, and a monster will get you."
Atheists do not assume there is a treasure, nor do they assume there is no treasure. Most atheists aren't looking for treasure, because atheists, if or when they think about the treasure at all, point out that nobody's ever seen the treasure, none of the maps are trustworthy, and that nobody's ever come back from going to look for it.
If you have a treasure map in your box, then more power to you. As long as your map isn’t telling you to force others to go with you on your treasure hunt, then as far as I’m concerned, it’s a private matter.
But my box is empty, the way it was made. There is not a treasure map in it. I do not, nor do any atheists, have a map that points to no treasure.
But I do believe in the existence of lugnuts.