What stops people stealing content and re-posting it on Steem?

Just a quick question from someone who wants to contributor quality, original content to this amazing platform... what stops someone from going through Reddit / Facebook / Instagram / Imgur / etc, stealing the top content and re-posting it on Steemit?

I'm pretty sure I've already seen a few people do it already. I guess it's a common trait of social media, but it seems kind of sucky that it could define the content shared on Steemit.