It is really peculiar to recount the history of unauthorized music sharing. While Napster and its kin suffered a history bloodier than any Tudor, YouTube seemed to stand tall amidst an acid rain of lawsuits:
Apparently, if you slap a bunch of family album pictures to the sound of Abba, you’ve created new art and are not in direct copyright violation.
This gave YouTube the time to formulate an acceptable revenue sharing model which most labels accepted, possibly as a last resort in a foredoomed battle.
So today we are used to searching YT for music, simply because that’s the only player that didn’t fold in a decade. Additionally, this is the only service that is truely internationalized and free, unlike e.g. Spotify or Grooveshark. This is probably since YT expanded during the wild west of music copyright, and thus was able to put its foot through the door.