Well, it also depends on the medium. I think a lot of people were pretty gobsmacked about the result of the Katy Perry Dark Horse lawsuit.
Yeah, sometimes it feels like music copyright is a law unto itself! I tend to emphasise to my students that in practice the courts take rather different approaches to the scope of copyright protection in different mediums, as you say. "Art" has the least scope & protection, "Music" & secondary copyright in Phonograms the most, and literary works are in between.