Credit:
Rickrolling, in case you haven't been online between early 2008 and now, is the practice of tricking someone into opening the aforementioned video, and it's considered to be one of the most popular Internet memes of all time.
Yes, it's a popular and copyrighted song, but it's also been viewed millions of times precisely because it became such a popular online meme. How removing it from YouTube will help copyright owners is a mystery to me. Props to Neowin for noticing this one.