One of the major problems with songs is they often “borrow” from songs in competition or even the same style. Art that’s appealing to wildly different customers is given significantly more leeway where art that competes head to head is more suspect.

This is one reason comedy is given so much freedom. It’s much easier to argue South Park retelling a story isn’t economically harming the original creators. That isn’t to say transformative only applies in economic terms, but such things make the arguments a lot easier.

As an example of this, Blizzard actually provided significant help in creating “Make Love, Not Warcraft” which shows just what transformative reuse can look like.