That atheists in the US sense a more extreme degree of stigma as compared many other regions of the world is a fact I can support with my data:
As is the case in any research, the "what" question is much more easy to answer than the "why" question. My goal in this post is to think through some possible explanations for the wide disparity between the US and other "Western" regions of the world, particularly Europe.
So far in my reading and thinking there are at least two major explanations regarding why religion is more dominant in the US than in Europe (and hence atheism is much more stigmatized).
First is the impact on the Cold War on the US. I submit the proposition that absent the Cold War and how it manifested itself in the American culture (McCarthyism and the other extreme reactions) the US would be far less religious than it is now. As a footnote to this period of US history, one contribution to the negative views of atheism might be the personal style of the most public atheist of the late 50's and 60's, Life magazines "most hated woman in America", Madalyn Murray O'Hair. The Cold War pitted the US against the “godless communists” and hence put on steroids the fairly mild levels of stigmatization of atheists and turned being a non-believer into something inherently anti-American and evil.
It seems obvious to say that the addition of "In God We Trust" to our paper currency was an artifact of the Cold War. A deeper look, though, reveals that we have had those words on our coins since 1864, with these additional words being added to paper money in 1957. Absent the Cold War would this have been done? Perhaps not.
A second major factor is, paradoxically, our constitutional separation between church and state. One arguments is that in Europe there are “lazy denominations”, that is, religion has been historically tied in with the state (the UK is a classic example) and hence have not had to work as hard for support; monopolies tend to get complacent. In the open marketplace of the United States, religions have had to fiercely compete for adherents and hence have after the hearts and minds of the public rather aggressively, yielding fuller pews and collection plates as compared to our European counterparts.
The above does not completely explain why atheism is more stigmatized in the US as compared to the rest of the Western nations, but I am convinced that this is a topic worth further study.