If you're able to write an episode, then you should do so. I can give my opinion of how good a movie or TV show is, but I don't have the ability to make my own movie or TV show. But I do have the ability to make a Fiction Branches episode. As does anyone who logs in to this site. It doesn't require anything but a certain amount of imagination and the ability to type.
Whatever the reason, it still shows that the person thinks that they can do better. And since they're logged in (you have to be logged in to rate episodes), why not show that you can do better? I'd hardly agree that writing an FB episode would be the same as producing a full-length movie. Writing an FB episode would be child's play by comparison. But I guess some people just can't do simple things.
Of course, it could also be that someone is giving low ratings like that because they're just a bunch of dumbasses that think that they're being hilarious, but aren't. In that case, I have a feeling that they don't even read the episodes. (Are they able to read at all?)