It is impossible to effectively monitor the huge volume of videos that are out there. It is often difficult to find out who owns the copyright on individual videos. Differing copyright laws in different countries also make the whole process harder.

By augmenting the pages in the upload process with educational text regarding the type of content that can be uploaded to YouTube, we have seen a sharp overall reduction with users uploading copyrighted materials.

What we have noticed at YouTube is that many users who have uploaded infringing content are unaware that it's illegal to do so.

Steve Chen Quotes