When a YouTube user posts a video from something they shouldn't, such as a copyrighted television episode, the third party who owns those copyrights contacts YouTube, who then removes the video and may also ban the user. It's a lose-lose: effort from the third party, wasted efforts and frustration from the user (and those who wanted to watch the video as well), and effort from YouTube in playing the role of po-po.
But now YouTube has implemented a win-win: when a user commits this felony, instead of YouTube punishing the user and sending the video in the black abyss, they coat the video is advertisements, and then split the ad revenue between themselves and the third-party whose copyrights are violated. The user can upload freely, other users can watch guilt-free, and YouTube and third parties can swim in pools of cash.
Given that the premise is so simple, it's strange this wasn't tried sooner. But better late than never, as YouTube internal conferences are no longer riddled with lawyers and fretting execs, and uploaders have greater freedom on the site than ever before.