IMO there’s a difference between writing a negative review in the comments of someone’s fic and what you do. I’m biased because I also review fic, so clearly I’m not going to tell you it’s wrong, but I think that if we take fic seriously as a form of artistic expression, even if it’s not a profit seeking one, it makes sense to analyze it and think about it critically. In the same sense, I think critical writing about visual art is interesting, but I wouldn’t encourage anyone to go to a gallery opening and analyze the strengths and weaknesses of an artists’ work to their face.