If you are informing me that a piece of fiction is, in fact, fiction, rest assured that I have a firm grasp on the difference between nonfiction/documentary and fiction, and on the different modes that exist between them.
But mostly people who say that are not trying to make that clarification with someone they suspect of being genuinely confused. They are saying, Relax. It is only a work of fiction and therefore not important. It is nothing to get excited over. It is nothing that can change your life or anybody else's, much less the world at large. It is certainly nothing for which you should have standards. It does not matter. It cannot matter.
I think that's false and not only that but damaging in its falsity, and while we're at it let's not forget patronizing, and I just plain don't want to hear it. Ever. Okay? Okay. I certainly don't want to hear it as many times as I have this week. I have gone over my quota.
I think that should get my crankiness for the morning out of the way, but I make no guarantees.