Why do so many atheists think that other atheists "ought" to be a humanist, or

Why do so many atheists think that other atheists "ought" to be a humanist, or

Why do so many 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? Isn't this both a psychological cognitive dissonance and a philosophical naturalistic fallacy? I think even David Hume would consider a religious person be more inclined to support those causes since they have sacred and textual norms to follow whereas the atheist has none.

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