Because continuing to engage with her content is a form of endorsement of her viewpoints (As mentioned in my previous comment, it is not inherently monetary support).

By saying, ”I enjoy the content. I don't support her views, but I'm not going to go out of my way to condemn her, and I'm going to continue to engage in positive discourse about her work.", you are indicating "I don't stand against trans bigotry unless publicly pressured to do so. I value the entertainment of her work more than dislike the harm my turning a blind eye to the harm the author causes with her hate speech." Most commonly followed by "I'm just going to pretend it's okay because it makes me uncomfortable to think otherwise. I'm not a bad person, and refuse to even acknowledge that I might be wrong here."

This simple line of thought that is understandably easy to fall into is often used amongst outright hate groups, transphobes in this case, to empathize with people within this community.

With that empathy built off of a mutual dislike for the trans people making them feel bad for just wanting to like their little HP fan club, they form a bond of an us vs them mentality, where the "us"becomes less and less about their love of the content, and more and more about their dislike of the " other."

Part of this discourse I'm engaging in here isn't in the hope that somehow I can dissuade anyone from joining this community. People gonna do what they're gonna do. But pointing out the potential pitfalls of founding and perpetuating this community is meant to instill in those that join an awareness of exactly WHY trans people and their allies are so upset that people are willing to turn a deaf ear to their voices when they tell you to think more carefully on this, to reconsider your position.

then it would seem to me that they have enough self-awareness to take care of themselves and mitigate any of this nebulous harm.

They don't though. Because to mitigate the harm would mean having a more difficult discussion on how JKR's works and awkward stumblings around inclusivity in her works have been nothing but tokenism. A facade of inclusivity made in bad faith. But that's not what this community would ever want to do, because to do so would be to point out exactly what I've been arguing this entire time, that to ignore the fact thst you are celebrating the works created by a bigot means you silently are endorsing her, even if you vocally condemn her.

People claim to care, and heck, words matter. But words only matter inasmuch as what those words do, or inspire people to do. JKR's words obviously have inspired a lot of people in a lot of different ways. But what have they ulimately inspired people to do really?

At best, they inspired people to get together and engage in conversations about a fantasy world, maybe come up with amazing stories of their own. At worst, they inspired some people to go out and make some poor trans person want to kill themselves.

The question I pose is, does the good really outweigh the bad? I don't think so.