Because seculars/athiests often believe that they're superior to the "stupid, God-believing religious" people, since their beliefs are obviously based on "pure logic and reason".
Yet, when you boil down anyone's value system to its fundamental essence, it turns out to always be a religious-like belief. No human value is based on pure logic, and it's annoying to see someone pretend otherwise.
> Someone is not good or bad because you agree with them
Right, that's what I was arguing against.
> Even a religious person can recognise that an atheist doing charitable work is being good
Sure, but for the sake of argument, I'm honing in on the word "good" here. You can only call something "good" if it aligns with your personal value system.
> The attitude you describe is wrong
You haven't demonstrated how. Could just be a misunderstanding.
You don't get to co-opt everybody as cryptically religious just because they have values.
That is not what a religion is.
> Secular Americans are annoying because they believe they don't have one
Why is that a problem to you?
> and instead think they're just "good people", calling those who break their core values "bad people".
No, not really. Someone is not good or bad because you agree with them. Even a religious person can recognise that an atheist doing charitable work is being good, regardless of whether they share a specific set of belief.
The attitude you describe is wrong, and from my experience much more common in religious fundamentalists than radical atheists (the vast majority of people in western democracies do not care whether you have a religion). I have never seen an atheist saying that. But I’ve had priests telling me that I had not "rejected Satan" because I was not baptised.