Although really, what would the point be of downvoting a submission? The most popular ones will still reach the top. I guess it just means that popular posts that cause a lot of division of opinion are able to reach the top. So something about Abortion that may normally be voted for neutrally (not saying it would, just an example), and is of a good quality can get to the top of the main feed.
Guess I missed that in the FAQ - but it's still a little bizarre. It's a fundamental failing (I think) in the majority of social networks (facebook likes, g+ +1s, twitter RTs, etc) - once something hits a certain level of "popularity", it can't become "unpopular"
"Why don't I see down arrows?
There are no down arrows on submissions. They only appear on comments after users reach a certain karma threshold."