Or at least they hold the opinion that the "God" position lacks credible justification. Sure, many/most atheists are pretty relaxed and simply don't believe in 'God'. I include myself in that group.
But a certain percentage of atheists do push their non-theism much further along towards broad anti-religious passion. Some atheists are as consumed by their anti-religious passion as the fundies are consumed by religious emotion. These kind of atheists don't just placidly lack belief, they are angry, dismissive and occasionally rather aggressive.
And what's wrong with that? Why should atheists be placid in their belief? You do understand that American atheists live in a country where we are surrounded by religious iconography in public space, to the point where our children are asked to pledge allegiance "under God" at school, and homosexuals are barred from the institution of marriage because of religious opposition. And this says nothing of the multicultural nightmare you find in parts of Europe.
Why on earth should atheists remain silent?
And more subtly, some atheists just seem to conceptualize religion as if it was Christianity, and Christianity as if it was a Protestant-style Biblicism. You'll find a certain kind of atheist studying their Bibles just as intently as the Christian fundies do, and reading them in the same highly literalistic way. There's a similar hostility to allegorical interpretation, a similar disinterest in non-Christian religiosity and blank incomprehension towards the Christian contemplative traditions.
I don't think that's true. I think there's an emphasis on Christianity in the US because that's the dominant religion, and the one that threatens equality. It doesn't have anything to do with people lumping all religion in with Christianity, it's simply a matter of atheists taking aim at the most prominent and hostile religions.