Some YouTube videos include a special label: "contains content from [link to YouTube channel]". Besides the unfortunate combination of words, it's interesting to notice that YouTube starts to fix some of the problems with copyright infringements by being more friendly with those who own the copyrights.
From YouTube's help center: "There are three usage policies -- Block, Track or Monetize. If a rights owner specifies a Block policy, the video will not be viewable on YouTube. If the rights owner specifies a Track policy, the video will continue to be made available on YouTube and the rights owner will receive information about the video, such as how many views it receives. For a Monetize policy, the video will continue to be available on YouTube and ads will appear in conjunction with the video. The policies can be region-specific, so a content owner can allow a particular piece of material in one country and block the material in another"
So what would you choose between the three policies: block (no exposure and no money) or track (exposure, but no money) or monetize (exposure and money)? Obviously, many content providers decided to choose the third option. In addition to the exposure, they could also make money from ads or from links to music stores, such as Amazon MP3.