The American Atheists have existed since 1963.
This is how they define themselves:
Atheism is the lack of belief in a deity, which implies that nothing exists but natural phenomena (matter), that thought is a property or function of matter, and that death irreversibly and totally terminates individual organic units.
The identity they have created for themselves is someone who is anti-religious and scientific, this is something that is not as socially acceptable in the US as it is here in the UK. Just like the an atheist may find it hard to accept religion many religious people find it hard to accept atheism.
They believe that the government should be separate from the state.
The first amendment:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
They believe that prayer should not take place in schools
They comment on recent issues such as Cee lo Greens performance of Imagine.
Like any other minority group they don't want to be penalised for their belief.