To label something as taboo does not mean it never happens; it means that it is considered socially unacceptable to do it. People are actively discouraged from doing it, and there may even be unpleasant consequences for doing it. This is what people mean when they point out that criticizing religious belief is taboo today in the United States. It happens, but it is strongly discouraged. Some of us think that needs to change. If there was anything new about the so-called "new atheism" aside from the fact that the mainstream news media decided to pay attention to it, it might have been that they pushed this notion more than most had been doing previously. This struck many of us as a good thing because we were eager to see the taboo fall.
For many Christians in the United States, the taboo against publicly criticizing Christianity is vital to maintain as one example of the privilege they enjoy. They recognize that some of it is inevitable as long as they are not empowered to punish blasphemy, and they prefer to incorporate it into their persecution narrative. They point to the times when their beliefs are criticized as evidence of mistreatment. And many atheists, including this one, often go along with them and refrain from criticizing their beliefs in many contexts.
We are used to questioning all sorts of claims, but we tend to make exceptions when it comes to those rooted in religious traditions. Many of us have internalized what we were taught about the taboo nature of criticizing religious claims to the point where it still seems unnatural to do so. I have been an atheist for many years, but I still catch this in myself periodically. And even those lucky enough not to have internalized this stuff often hold our tongues to avoid conflict or punishment (e.g., criticizing religion in the workplace may lead to sanctions).
I think there are plenty of times in plenty of different situations where an atheist hears a religious believer say something absurd and feels the rising impulse to say, "bullshit" but exerts self-control to prevent from doing so. I'd never claim atheists were unique in this respect. I've known plenty of Christians who describe a similar experience when interacting with non-Christian religious believers or those who express political views different from their own. What I would say, however, is that atheists living in the United States probably have this experience far more often than their Christian neighbors.
We hold our tongues for many reasons, and it is important to acknowledge that some of them may even be good reasons. If I am friends with a Christian and value our relationship, keeping some of my thoughts about what they believe to myself is easy to justify because it probably is helpful to maintain our relationship. And yet, taboos do not usually disappear on their own. If we want it to be less socially unacceptable to criticize religious belief, we probably need to do more of it. Others need to hear more of it so they will get used to hearing it, and that's how hearing it becomes less of an issue. By holding our tongues, even when we do so for good reasons, we are helping the taboo stick around longer.
Fortunately, criticizing religious belief does not mean that I must get in my friend's face and yell "bullshit" or call religious believers names on Twitter. I can start by explaining that I do not share their religious beliefs and telling them why. Not only is that an important place to begin, but it may also create space for a discussion in which they ask me to say more. And sure, I think it is perfectly acceptable for any atheist who encounters a religious believer talking about their beliefs to take that as an invitation to express what they think about what is being discussed, as long as they are aware that this can still be risky.
We are never going to inhabit the kind of world where anybody can say anything in any context without consequence. That's not the goal here. The goal is one of equality. As long as it is acceptable for a religious believer to discuss their religious beliefs, it needs to be acceptable for an atheist to discuss whatever they believe, including what they think about the religious beliefs being brought up.
