While no author deserves harassment, it's important to differentiate and separate the work from the author. The work can (and should) be subject to criticism, even harsh criticism, but unless the author is shady, I don’t think anyone should bash them. On the other hand, good works should be appreciated even if the author is a dick (I like Harry Potter but hate J.K. Rowling).Yeah, I feel bad for them now. Didn't even know all that was happening.
What some anons on Tumblr do is harass the authors of any story they dislike. Since the asks are anonymous, it's pretty easy to do so. For the most popular IFs, most assholish anonymous asks are ignored or buried under thousands of other messages, but in smaller ones the harassment sticks out like a sore thumb. And if the author responds, it creates a snowball effect, leading to even more harassment. The only way to actually reduce the amount of harassment is to turn off anonymous asks. But this is a double-edged sword, since many people don’t want their profiles to be associated with the ask.