Maybe they want to emphasize the purity of the original 'Harry Potter' story. They don't want it to be confused with the many fan - made stories out there.
Perhaps they don't fully understand the depth of the world - building. They might see all the magic and think it's just some random, crazy ideas thrown together like in a not - so - well - thought - out fanfic.
Perhaps they haven't fully understood the series. Some people might just skim through the books or movies and miss out on the depth. They could be focusing on just the surface elements like the magic spells and not realizing the complex relationships and themes beneath.
Perhaps they've come across some really strange fan - created stories. Some fanfictions can be very out there with their interpretations of the wizarding world.
Well, perhaps it implies that the essence of Harry Potter is strictly within the original works. Fanfiction often takes the characters in different, sometimes untrue - to - the - original - spirit directions. So, Harry Potter, as created by J.K. Rowling, has his own defined storylines and characteristics which are not related to the often unregulated world of fanfiction.
One reason could be that they feel the official Harry Potter story has a defined end, and the fighting in fanfiction is just over - done. They want to focus on the original tale's conclusion.
Maybe they just want to be controversial. Some people like to make extreme and untrue statements to get attention.
Maybe they just don't understand the real essence of Harry Potter. They might have only seen a small part of it or misinterpreted some of the more emotional or character - developing moments as signs of weakness, which is completely wrong.
One reason could be copyright. The creators and rights holders of Harry Potter want to protect their intellectual property. Fanfiction can sometimes blur the lines of ownership and might use elements of the story in ways that aren't authorized.
Perhaps they have a distorted view of creativity. They think that by adding such extreme and inappropriate elements, they are being unique. However, they fail to realize that it not only goes against the values of the Harry Potter series but also has a negative impact on the fans and the overall perception of the story.
Maybe because they are a big fan of the original 'Harry Potter' series and are tired of seeing really bad fanfiction. They want to emphasize that the real 'Harry Potter' is much better than those poorly - made versions.