Because if they don’t want anyone to know that they have an AO3 account, I’m not gonna be the one who points it out. Since I don’t know their feelings on the matter, I’m not mentioning their name or the fic.
Okay, let’s try a hypothetical what-if scnario: Say as an author, I use my real name for a tumblr handle (which I did, but just bear with me here). Now, my real name is fairly common, so it’s not as if I would be outing myself unless I announced it on the tumblr. It’s not necessarily a huge secret, but I don’t tell people.
Then someone comes along and points out that tumblr and shouts about who that REALLY IS. If I wanted to remain on the downlow, I’d not appreciate that a whole lot. I’d deal with it, because there is no such thing as being truly anonymous on the internet without going through a great deal of pain in the ass trouble, but if remaining quiet had been my goal, it would suck to deal with the fallout.
So that is why it is hypothetically rude to go running and pointing when someone who is famous says something under an account that may not be public knowledge.
It has also been pointed out to me: Stalkers are a thing and they suck and I’m not gonna give someone with those inclinations an easier time of things.