Why do so many atheists think that other atheists "ought" to be a humanist, or
I am far from convinced that a particularly large number of atheists think that other atheists "ought" to be a humanist, or at least care about animal rights, environmental protection, left-wing politics, and so on. Do you have any evidence for this idea? I am also puzzled by the suggestion of a naturalistic fallacy. You seem to be attributing to this possibly imaginary large number of atheists some kind of argument that they are supposed to put forward. But you do not say what it is. There are many religions and many religious people and many different sacred and textual norms saying or implying completely different things about animal rights etc., many inconsistent with others. And these are followed in many different ways by different people. Atheists do not tend to follow sacred texts. But there are large numbers of texts about about animal rights, environmental protection, left-wing politics, and so on that have nothing whatever to do with religion.