HDC. Is. A. Competition.
Yes, it's a happy funball fandom competition, but it is a competition. Not a gift exchange. Not a charity drive. Readers are invited to judge the stories on their own merits, to vote and comment based on their impressions. This is not a secret and it has not been a secret from the beginning of the fest.
The whole premise is that the stories for each prompt will compete with other stories for the same prompt, and in the end, the team with the most votes for all prompts is going to win a bunch of Photoshopped banners. It isn't serious business. But it's a competition all the same, and in a competition, the judges (in this case, the readers) have a right to say if they didn't like something. Please get the fuck out of orbit and back to Planet Earth; you're blocking the sunshine.
It's a very fair point to make that if someone liked absolutely nothing about a story, it's quite tactless to leave a comment on the story telling the author so. The author can't really reply, for one thing, at least not as themselves. If I don't like a story as I'm reading it, I'm not going to waste my time finishing it and leaving the author a scathing comment, berating her for wasting my time when really I wasted my own damned time by persisting to read something I wasn't enjoying.
However, I haven't yet seen any comments that are pure 100% negativity. Even if such comments existed, the reader would still have had a right to leave them, as long as they weren't obnoxious, attacking the author's beliefs based on what they glean from the story, or whatever.
It isn't polite and it's ultimately pointless -- a writer can't please everyone, and if you aren't pleased halfway through the story, you should have the presence of mind to, you know, stop reading. But that's my opinion, and though I know it's shared by a fair few people, I don't think it ought to be shoved down anyone's throat. Not everyone thinks leaving bad feedback is rude or pointless, and some people who don't think so even have personally valid reasons for it.
One person brought up the point that in HDC, not reading a story through to the end makes it unfair to vote, because you really shouldn't vote on a story you haven't read. I do sympathise with that stance, but if you're putting yourself through misery reading a story you disliked two paragraphs in -- just so you can vote for it in good conscience -- it is your problem. So leaving fully negative feedback for the author is still rude, I think, but that is my opinion, take it or leave it.
The reader/writer relationship goes two ways. We the writers are not gifting our incredible talents to the unwashed masses. We are sharing stories with readers who are in the same fandom, who love the same characters/pairings, who have opinions that might differ from our own but are no less valid just because the reader might not be a writer. We are not entitled to special consideration because we are ~*~WRITERS~*~. Readers are also people, and telling readers to STFU if they aren't full of unmitigated squee is a giant "fuck you" to the readers, whether you want to admit it or not: it devalues the reader's opinion and places the writer above the reader on the metaphorical fandom ladder.
This is why we can't have nice things. And also why I'm glad I've adopted a personal policy of not commenting to anything until after the reveals, because surely if I fail to squee 100% in one comment or another, I'll be accused of hating freedom. Again. -_-