As stated before, most myths about Atheism seems to stem from a lack of knowledge about the simple definition of Atheism. However, there are times when the definition does not even factor into the myth. In fact, some myths seem to come from what could almost be described as jealousy. It's kind of strange, really, especially since there is nothing to be jealous about.
Here's one that I get a lot when I discuss my atheism online.
"You're just an atheist so that you can do whatever you want!"
Actually, that's not true at all. Being an Atheist means we answer to the laws of humans, not anything or anyone else. We are limited to what the majority of humans want us to do. Sure, some people like to break the law, and there are those that like to think the ones that got away with it are punished after they die, but life isn't that fair.
One major part of this myth comes from an earlier topic I discussed, the claim that Atheists don't have any morals. As I said then, and will restate now, that is far from the truth. Atheists have a great deal of morals, usually the same ones Theists have. We simply tend to believe science when they say our morals stem from the evolution of the brain and majority opinion within society.
The other part, in my opinion, stems from a certain jealousy. A Theist that makes this claim may actually be a little jealous of the Atheist's supposed freedom. However, they are just as "free" as we are, they just choose to see it differently.
Again, with any myth, there is a bit of truth to the tale. Some people who become Atheists are leaving religions that restricted them in many ways. As a passive example, Jews are not allowed to eat pork products. As such, a Jewish person becoming an Atheist might be allowed to eat all of the pork they want. They would be allowed additional freedom they did not enjoy before. As I stated, though, this is simply a way of looking at things. The Jewish people have always been capable of eating pork, they simply choose not to based on their beliefs. The freedoms are there, it is simply a matter of whether or not you choose to accept them.
As for the core part of this myth, freedoms, whether perceived or otherwise, are not the reason most people become Atheists, it is merely a perk. Most Atheists are either born Atheists or become them through a lack of belief. The freedoms noticed might be a factor in their beliefs changing, but it is the change in beliefs that is the true reason Theists become Atheists.
No amount of freedoms can change your beliefs.