It's possible that he hates it because in fanfiction, his friends and he himself might be portrayed in ways that are completely different from who they really are. For example, Hermione might be made into a silly character instead of the intelligent and brave witch she is. This distortion of the people he cares about could be a big factor.
Naruto might dislike fanfiction because it can be a source of misinformation. Fans who read fanfiction might get the wrong idea about him or his world. He wants people to understand his story as it was originally intended. The original Naruto story is a carefully crafted narrative about ninja life, dreams, and the pursuit of peace. Fanfiction can deviate from this and make it seem like something else entirely, which he would find unacceptable.
Maybe Percy has a different set of values. He might be more into peace and stability rather than the chaos and violence often associated with the hunt. In fanfiction, the hunt could be depicted in a way that goes against his moral code.
She may not like the false interpretations of Batman in some fanfictions. Fanwriters might change his personality or motives, and as a fellow hero, she doesn't appreciate that.
Tony Stark may dislike Avengers fanfiction because it can mess with the emotional core of the team. The real relationships between the Avengers are deep and complex. Fanfiction might simplify or distort these relationships. For instance, his friendship with Steve Rogers has many layers, and fanfiction could make it into something that doesn't do justice to their real bond. Also, he's a man who values his privacy to an extent. Fanfiction might delve into his personal life in ways that he doesn't appreciate, making him feel exposed and misrepresented.
One possible event could be that a powerful dark magic spell was cast on someone he loved, and he couldn't reverse it. This would make him see magic as a dangerous and uncontrollable force, thus leading to his hatred. Another event might be that he was wrongly accused of using dark magic, and the whole wizarding community turned against him because of it.
Perhaps in the fanfiction, the idea of Harry rejecting magic is a form of rebellion. He might have seen the negative impacts of magic on the wizarding world, such as the power struggles and dark magic. So, he decides to take a stand against it. Another possibility is that the writer wants to show how much of Harry's identity is tied to magic and by rejecting it, he goes through a major transformation.
In fanfic, Percy could hate the gods because of their lack of accountability. For example, when a monster attacks a demigod camp, the gods don't always step in to help. Percy may have to fight and risk his life while the gods seem content to watch from afar. Additionally, the gods' treatment of their own children can be quite cruel in some interpretations. Some demigods are abandoned or given impossible tasks, and Percy might resent the gods for this behavior towards his fellow demigods.
In some fanfictions, dark magic interference might cause a wand to explode. If a dark wizard tries to curse a wand or disrupt its magic, it could lead to a catastrophic failure. This could be part of a plot where a hero's wand is targeted by an enemy.
Perhaps they've received some negative feedback related to certain fanfictions that crossed the line in terms of content or ethics. So, in order to safeguard the reputation of the Harry Potter brand, they've decided to put an end to fanfiction. This would ensure that the official image of Harry Potter remains intact and positive.
Perhaps he likes to see how fans re - imagine his character. He might be curious about how others view his actions and motives throughout the story.