That's nowhere in the Hacker News Guidelines.
http://ycombinator.com/newsguidelines.html
You are, of course, entitled to your opinion about how to use downvoting, but other users here are entitled to theirs.
Here is what pg said about the matter 1280 days ago:
http://news.ycombinator.com/item?id=117171
"I think it's ok to use the up and down arrows to express agreement. Obviously the uparrows aren't only for applauding politeness, so it seems reasonable that the downarrows aren't only for booing rudeness."
There have been refinements in the site software since then, notably the change of a few months ago to hide individual comment karma scores from readers other than the comment's individual author, but there has never been a statement from site management that downvotes must necessarily be explained. Sometimes the best way to respond to a comment that is not thoughtful and well informed is to downvote it.
When pg wrote his post earlier this year "Ask HN: How to stave off decline of HN?"
http://news.ycombinator.com/item?id=2403696
He wrote, "The problem has several components: comments that are (a) mean and/or (b) dumb that (c) get massively upvoted." By implication, some comments don't deserve any upvotes, and may indeed deserve downvotes without further subcomments.
And your second example makes better sense than your first. None of those three reasons have to do with agreeance.
Downvoting causes comments to literally disappear. It's tragic that good comments disappear because some people disagree. Those are frequently the most valuable comments.
Downvoting should be reserved for obviously low/no value comments. It adds no value to downvote comments merely because they express that Groupon might have a viable business.
It's your flawed assumption that everyone else is wrong and you're right.
Groupon may or may not be a viable business.
However, your assumption is guiding you and you haven't provided enough data or facts. Your opinion doesn't really matter or your annoyance that everyone is picking on poor Groupon.
There are a lot of red flags and you are avoiding mentioning them and not mentioning very much supporting data for your assumption.
We aren't here to listen to wild conjecture.