If I can take a swing, I think the difference between American and EU (at least, UK) atheists is due to the respective theist groups. As far as I understand, while a large portion of the EU is religious to some extent it is far less common for religion to be the beginning and end of someone's character. And, despite our "Wall of Separation" in the states, it seems we have a lot more religious rhetoric and invocation in politics than the EU/UK. (Disclaimer: I don't follow politics in Europe closely.) Yes, the UK might have "God Save the Queen", but that doesn't seem any different than "God Bless America": it's rarely a religious invocation, just an expected stump line and a way to end a speech.
So the relative pervasiveness of religion in America, especially with things like the "War on Christmas" fueled by the likes of Fox News and Rush Limbaugh, means that many religious folk don't see atheists as some fellow citizen with a different perspective but as a an all-powerful enemy seeking to destroy their religious beliefs. This mostly imaginary threat puts them on the offensive towards theists, and some atheists--perhaps not even most, but a noticeable portion of American atheists--have gone on the defensive. You can look up things like "atheist congregations" and such Defense Leagues that are, as far as I can tell, simply about meeting other people without having to hear "Have you found our Lord and Savior Jesus Christ?" (yes, he's in my closet, hiding from you) and having power in numbers.Some of it may also be those who have recently ended their faith but who are used to and enjoyed the sense of community, or maybe feel a hole without a tight community to belong to (not that this is inherently a bad thing). Though an atheist, I've not sought these out or read much about any specific group, so I could be way off here.
 I'm sure there are some atheist groups who seek complete annihilation of religion in America, but they're as representative of atheists in general as the Westboro Baptist Church is representative of protestants. Myself and atheists that I know don't give a shit so long as religion isn't being crammed into government or schools.