I think a simpler explanation is just that people are not absolutists about following social norms, so they'll regularly violate a norm if it comes into conflict with another norm or something else. To take one example, there is a clear social norm against lying which children learn (they are told not to lie and chastised when they are caught lying). But people still lie all the time, and not just for personal benefit but also to spare other people's feelings and, perhaps most commonly, to make social interactions go more smoothly. And instead of seeing these cases as violating the norm against lying because something else is even more important here, it seems like liars often don't even feel like they are breaking a norm against lying. Instead, the norm against lying doesn't even get applied to this case.
How do people manage to pull off this flexibility in applying norms? The main trick may be something as simple as: once you've decided on something and have a norm that matches your decision, other norms are irrelevant - there's no need to even consider them. Although that leaves open the important question of how one norm wins in the first place. (Another possibility is that people are using something like modus tollens: lying is wrong, this is not wrong, therefore this doesn't really count as lying.)
Eliezer and many others here are absolutists about the truth norm, but most people see it as on par with other norms, like the norm in favor of being upbeat or optimistic and the norm about people being entitled to their beliefs. And when norm absolutists run into people who are mushy about their favored norm, they may doubt that those people even have the norm.
Followup to: Applause Lights
When you say the word "truth", people know that "truth" is a good thing, and that they're supposed to applaud. So it might seem like there is a social norm in favor of "truth". But when it comes to some particular truth, like whether God exists, or how likely their startup is to thrive, people will say: "I just want to believe" or "you've got to be optimistic to succeed".
So Robin and I were talking about this, and Robin asked me how it is that people prevent themselves from noticing the conflict.
I replied that I don't think active prevention is required. First, as I quoted Michael Vassar:
But more importantly, I don't think there does exist any social norm in favor of truth. There's a social norm in favor of "truth". There's a difference.
How would a norm in favor of truth actually be expressed, or acquired?
If you were told many stories, as a kid, about specific people who accepted specific hard truths - like a story of a scientist accepting that their theory was wrong, say - then your brain would generalize over its experiences, and compress them, and form a concept of that-which-is-the-norm: the wordless act of accepting reality.
If you heard someone say "I don't care about the evidence, I just want to believe in God", and you saw everyone else in the room gasp and regard them in frozen shock, then your brain would generalize a social norm against self-deception. (E.g., the sort of thing that would happen if a scientist said "I don't care about the evidence, I just want to believe in my-favorite-theory" in front of their fellow scientists.)
If, on the other hand, you see lots of people saying "Isn't the truth wonderful?" or "I am in favor of truth", then you learn that when someone says "truth", you are supposed to applaud.
Now there are certain particular cases where someone will be castigated if they admit they refuse to see the truth: for example, "I've seen the evidence on global warming but I don't want to believe it." You couldn't get away with that in modern society. But this indignation doesn't have to derive from violating a norm in favor of truth - it can derive from the widely held norm, "'global warming' is bad".
But (said Robin) we see a lot of trees and hear the word "tree", and somehow we learn that the word refers to the thing - why don't people learn something similar about "truth", which is supposed to be good?
I suggested in reply that the brain is capable of distinguishing different uses of the same syllables - a child is quite capable of learning that a right turn and the right answer are not the same kind of "right". You won't necessarily assume that the right answer is always the one printed on the right side of the page. Maybe the word "truth" is overloaded in the same way.
Or maybe it's not exactly the same, but analogous: the social norms of which words we are meant to praise, and which deeds, are stored as separately as left hands and leftovers.
There's a social norm in favor of "diversity", but not diversity. There's a social norm in favor of "free speech", but not pornography. There's a social norm in favor of "democracy", but it doesn't spontaneously occur to most people to suggest voting on their arguments. There's a social norm in favor of "love", but not for letting some damn idiot marry your daughter even if the two of them are stupid and besotted.
There's a social norm in favor of "honesty". And there are in fact social norms for honesty about e.g. who cut down the cherry tree. But not a social norm favoring saying what you think about someone else's appearance.
I'm not suggesting that you ignore all the words that people praise. Sometimes the things people praise with their lips, really are the things that matter, and our deeds are what fail to live up. Neither am I suggesting that you should ignore what people really do, because sometimes that also embodies wisdom. I would just say to be aware of any differences, and judge deliberately, and choose knowingly.
Sounds good, doesn't it? Everyone knows that being "aware" and "choosing knowingly" must surely be good things. But is it a real norm or a fake norm? Can you think of any stories you were told that illustrate the point? (Not a rhetorical question, but a question one should learn to ask.)
It's often not hard to find a norm in favor of "rationality" - but norms favoring rationality are rarer.