Well not all comments are helpful. I deleted comments because they're very obviously bots or would be scammers. Also people could say unrelated things on there too. Like discriminatory comments or personal insults. Putting your content on here doesn't mean you deserve to be one-sidely harassed.
It's one thing if you are giving honest feedback. But if it's insulting or bordering on insulting, then there's no need for that. If someone is erasing just feedback, then that hurts them. It won't hurt the readers if they don't see it, just the author. As someone who's read a lot of bad books, it's simple if I read a book that is bad but didn't give me warning from the reviews or comments. I just stop reading it. Or make an honest review or rating (without insults). It's that easy. You can be a reader without be a jerk.
As for reviews, while I'm new on here, it's my understanding that they don't remove those unless the website moderator thinks the review has broken the rules of the website.