They "don't know it doesn't make sense" because it does make sense, and that isn't what they're asking. You're misunderstanding the question they're asking if that's what you're taking from it. They're using the fandom definition of canon and not your definition; they're asking why a tie-in can't be included as official on the same level as the base material. They aren't asking about consistency because, as you said, canon material can be inconsistent as well.
People outside fandoms use it that way too. Joss Whedon, for example, used the word with that definition describing the comic series "The Origin" despite no one from the show having worked on it:
The origin comic, though I have issues with it, CAN pretty much be accepted as canonical.
Now, I'm definitely not using this as an argument that books should or shouldn't be counted as canon; I don't care what the "official" word is either way there. What I'm arguing is that the new definition of canon is supplanting the old. Many franchises outside Star Trek don't use the old definition of canon. I'd say most creators
nowadays outside Star Trek use the new definition of canon, even. And saying that anyone that uses the new definition of canon is wrong is akin to saying that ending a sentence with a preposition is wrong from a linguistic perspective.
I mean, heck, the "original" definition only even came about because some Sherlock Holmes fans misunderstood a joke other fans were making about how the books should be included as part of the Bible.
Edit: Just to clarify, when I talk about the new definition vs. the old definition for canon, I'm not saying what should or shouldn't count as canon; I just realized it could sound that way. I'm literally talking about what the word means, the old definition being "something created by the original creators" and the new being "something which in a given fictional universe is considered to have happened with regard to that fictional universe", which can be either by the creators of a given fictional universe (the so-called "official" canon, as little as that matters) or by an individual fan or fans of a given work, and unless otherwise modified (such as "personal canon", "headcanon", or the like) is usually taken to mean the former.