Atheists are all just cynics. They reject God because of a cynical distrust of what others tell them.
It's not common for atheists to be openly described as cynical, or at least inherently cynical, but this seem to be a common attitude which people have about atheists. When atheists appear on TV or in the movies, it's pretty normal for them to be cynical, broken people who have "lost their faith." God failed them and now they distrust everyone and everything around them. Thus this myth is closely linked to the myth that people are atheists after feeling that God didn't do something for them.
Such fictional stories are merely a reflection of the general attitudes in American society. Religious, theistic faith is a virtue and something that fills a person with the ability to love, trust, be generous with others, and live a happy life. When a person doesn't just lack that faith but has consciously rejected it, this means that they must also be rejecting the ability to love, trust, be generous, and live a happy life. What kind of monster would be like this? It's a monster that goes by the label "atheist" and the adjective which describes such attitudes is "cynical."
Cynicism was a school of philosophy in ancient Greece founded by Antisthenes. The Cynics lived ascetic lives in which the attainment of virtue was the only goal because virtue itself was believed the only thing necessary for happiness. Cynics didn't care what others thought about them and weren't interested in social norms. Because of their strict lifestyle, they were widely ridiculed. The connection with modern cynics is largely in the disdain for social norms and rituals that serve no apparent, practical purpose.
This is not to say, of course, that no atheists are cynical. If a person becomes an atheist after many years of religious theism, it's possible that they could become very cynical if they feel that they have been lied to by parents, family, religious leaders, etc. It's hard to imagine how such a situation wouldn't cause a person to become at least a little cynical and it's difficult to blame someone who ends up this way. The cause isn't atheism, though, it's how a person reacts to what they learn about what they have been taught about religion through their lives.
It's also true that atheists tend to be critical about many social norms and traditions perhaps more critical than the average theist. Once again, though, this isn't something that follows automatically from atheism itself but instead from the fact that atheists in America are already somewhat at odds with society's dominant belief system: Christianity. The Christian religion is an important means by which tradition, morality, norms, and behavior are structured in America. Once a person has rejected that, they are in a position to become far more critical and skeptical of all the norms and traditions which have been propped up or justified by it.
Thus if we simply define being "cynical" as being skeptical or distrustful of popular norms, traditions, and beliefs, then it is true that atheists in America tend to be cynical. Atheists in countries where they haven't gone through the same process, and thus haven't had to adopt a more critical attitude towards the dominant belief system, won't be nearly as "cynical." This definition is misleading, though, because it tends to connote more than mere skepticism, including the very negative attitudes described above. Atheists shouldn't accept the label "cynical" based on this second and neutral definition if it's only going to lead to people apply the negative definition above.
It's certainly wrong to depict all atheists as being necessarily distrustful, joyless, or loveless. Some are, but some theists are as well. It seems to me that the assumption that atheists are generally cynical stems from misunderstanding and prejudice against atheists. Deliberate promotion of this myth is a means by which religious theists an dismiss atheists, ignoring their critiques of religion and especially the way they challenge the status quo. A person who is cynical and thus distrustful of people and norms may be assumed to not have any special reasons to criticize any social institution in particular they criticize everything, after all. That, however, is just a weak way of avoiding those challenges and pretending they don't exist.