They're welcome to set whatever rules they want for what's "on topic" or a "good fit". Their site, their rules. No harm, no foul.
It's just irritating as hell to waste your time clicking over there from a Google link and finding a closed discussion at the other end.
Telling Google to not index a topic would be a minor tweak. If they've found the discussion unworthy, why do they want Google to index it anyway?
Perhaps on some level SO knows this, and also knows that their mods are perhaps somewhat overzealous, and therefore has adopted the current state of affairs as a sort of compromise/duct-tape solution.
I too, have found so many downmodded questions to have the exact answer I needed for a particular problem. It's by far the most frustrating part of using Stack Overflow.
The overzealous modding has had a chilling effect on my desire to ask a question on SO, because nobody wants their question to be downmodded as a stupid or pointless question.
I find some of the moderation really frustrating but it's worse when the people closing threads and answers are also the ones leaving zero-content comments.
The good thing about Stack is anything I write there will be as easily accessed as if I'd posted it on my own blog, not re-purposed as SEO-bait and blurred or hidden until the user signs up. Furthermore, it's licensed under Creative Commons instead of serving as a donation to the site owner.
Quora is actually a great site once you're logged-in and I find a lot of the content fascinating, but I also wonder why people continue to contribute to such a closed system.
It isn't the perfect model but it is certainly a better one.
Despite what you claim as lacking rewards it seems to have thrived and attracted contributions. I don't really care what you consider a good community, or even accept the premise that there needs to be any notion of community. Why attribute labels, the model seems to work just fine.