I don’t know these tags, because it’s rare that I come across these kinds of fics, but I will try to address this. This might end up being long, so I’ll put a cut.
I think in big part, because Tumblr is an echo-chamber and a circlejerk. People are encouraging each other (whether it be good or bad).
Also, because consuming and enjoying literature with mature themes and teenage characters is automatically a slippery slope to enjoying mature things with teens irl. I’m not saying that’s true or not. This is just why I think people freak out. You don’t know if an adult is reading these fics.
And tbh, idc. There are plenty of published novels with mature themes with teenagers and children in them. Sometimes, it’s just teens being teens, and sometimes, the writing is fucked up. And that’s the point. It’s like rubbernecking while driving by a tragedy. It’s engrossing, and you can’t look away.
People should read these things if they want, but with a critical eye. That is my opinion. For example, I read Lolita, and while it disgusted me, it also impressed me. The author made you almost sympathize with a pedophile. It shook me to my core. But I was aware that what the narrator did was wrong, and I could appreciate the effect the novel had on me. The effect being the almost-sympathy.
If someone is reading these things for the sake of reading about teens having sex and getting off on it, well that’s different, of course. And you may disagree that published novels and fanfics can’t compare in this context, but I do think these judgemental people will judge others for reading those, too.