Earlier quoted context omitted.
Well, that's kinda the thing. If something was in one of those history courses, then they were important enough that it mattered. Playing into the Santa Claus lie with your friend's kid is absolutely inconsequential to you, but it could mean a lot to that kid (and to the parents) to keep that lie alive a bit longer. (And if telling the truth to that kid, against what the parents would wish, is that important to you,…
> But if we're talking about a kid believing in Santa Claus, and the parents actually do care that you keep up the fantasy, you actually are an asshole if you tell the truth, no question. Why is the desire of the parents more important than the desire of the child? Remember that we're talking about a scenario where the child is explicitly asking. They want you to tell the truth, their parents want you to perpetuate a…
You may not see the useful function of the Santa Claus fantasy[0], but it is not your (or my) place to decide that for a child or their parents.
Let's try with something a bit less trivial, something that my young nephew had to deal with not too long ago: death. One day kid realizes that grandpa hasn't visited in a long while, and ask parent what's up. Parents don't think kid is emotionally ready to have the death talk. So they spin a story about grandpa having to move away, and how grandpa loves his grandkid very much, but won't be able to visit anymore. And let's say you're over visiting your friend, and their kid walks up to you and says "my grandpa had to move far away and I miss him... do you know why he can't visit me?" You know that your friend's father died recently. If your answer isn't "well, what did your parents say?" followed by affirming whatever the answer is, you've probably done a bit of harm to that child's emotional well being, and likely have introduced some trust issues between the child and parents. What right do you have to do that? I would not be surprised if your friend stops inviting you over after that.
Don't get me wrong; I think many decisions parents make for their kids probably aren't the best ones. There's no authoritative parenting handbook, and most parents do the best they can. But it is not anyone else's place to "correct" someone's parenting decisions, outside of active harm and neglect. (And no, telling a lie about Santa Claus or death does not clear that bar.)
[0] Personally I'm not a big fan of it myself; while Santa Claus can be a useful tool to help a child's imagination develop, I really dislike how tied Christmas has become to consumer culture and materialism.