That sounds classist to me, and exactly what I'm railing against. The use of "fan" to describe work that isn't profitable is gross w/o further justification, which is why using it to describe works by fans of an existing property is not. It arises not out of class but out of the concept of a canon.
I don't think you're actually willing to understand this, but, one last try: fanzines called themselves that. They originated the term. It was not an insult. It is, yes, considered an insult now, mostly by people who think fanfiction is icky girl stuff/too gay/too weird, but it is never a SLUR.
Me and my fellow white people call each other "white trash." That doesn't make it acceptable for an out-group to refer to novels by white people as "white trash books." It would, in fact, be a slur to do that. (I'm sure you've by now drawn the connection to other slurs used by in- and out-groups.)
Again, "fanzines" were and ARE called that by the people (the fans!) who make them. Fanzines were put together by people who had the time and disposable income to assemble a physical magazine for fun, not profit. There was no pejorative meaning associated; it's like calling people who knit knitters.
slur, noun: "an insinuation or allegation about someone that is likely to insult them or damage their reputation" It seems like this is the moment you learn what "slur" means.