"Fairies don't have a very efficient news system. Fairies don't always do things just because they want to. If you could reliably get names out of fairies, then some fairies would find out, others would remain in the dark, and some of them would cozy up to their rivals, suggest that those rivals go antagonize you, and use you to get the names. If they wanted you dead they'd send disposable vassals with no choice in the matter and stay safe and far away. Even in the paradigm case where some fairy decides to try to kill you of their own accord and turns up in person to do it, publishing their name opens them up to arbitrary abuse for as long as literally forever, especially if you write their name down - not that killing somebody isn't bad, not that a successful attempt wouldn't probably leave you dead forever, but throwing them to the whims of whoever catches them first when those whims could be anything at all is unnecessarily cruel. Fairy masters usually keep the option of retaining a written record of a name as an escalation option if they don't like how their vassals are behaving and aren't interested in fixing that with more refined orders."