Because someone posted it, and a bunch of people voted it up, and there's no downvote to express our displeasure with get-rich-quick junk or content-less Techcrunch stories or what-have-you.
There is. Flagging. And before you say it's "not for that", people are clearly using it as such. I've had perfectly legit posts (that I haven't "promoted" at all) slide way lower than their votes would indicate due to flaggers. So it seems a legitimate downvote system now.