It boils down to intent and practice.
If the intent is to steal content just for the sake of distributing it in its unaltered form, it’s an obvious example of theft. On the other hand if you take that content and create something new with it (the law may not see it this way) but it seems to be borrowing, and shouldn’t be punished.
In determining what is borrowing and what is stealing ratio seems to be important. What’s the percentage of original material compared to copyright material? How well does it resemble the source material?
There are many electronic musicians who sample bits and pieces of songs and works from other artists. This sampling is the foundation of plunderphonics.* I’m not talking about terrible mash-ups either. The songs produced however do not resemble their source material. There is a certain art to putting pieces together, which initially don’t seem to fit. Who’s to say that this artist is infringing on any copyright laws when they have created an original artwork?
Borrowing elements from other people and combining them in new ways is a great for new ideas and artworks to develop. Let’s equate it to collage. In its simplest form, we’ve all probably made a collage. Cutting out magazine pictures and using a glue stick to paste them on poster board is a simple example. The images that go up on that poster board are not yours, even though you paid for the magazine, and you certainty didn’t make them. You did, on the other hand, take the time to gather them and arrange them in new ways to make something new from the pieces. You have, from those disparate pieces, constructed new meaning and a new artwork. In law, the concept I am referring to would be called transformativeness.
Having loose copyrights laws allows this synthesis to occur. Others have mentioned Star Wars Uncut, which is a great artistic and creative endeavor even though it is based off preexisting material. People borrow others ideas all the time, and use them for building blocks for their own works. It happens in music, it happens in film, albeit in more subtle ways (except with Tarantino), the Internet just has a way of being less subtle about…well, everything.
“The public domain is like a vast national park without a guard to stop wanton looting, without a guide for a lost traveler, and in fact, without clearly defined roads or even borders to stop the helpless visitor from being sued for trespass by private abutting owners." - This Business of Music
*The terms was coined by John Oswald in his article Plunderphonics, or Audio Piracy as a Compositional Prerogative. Check it out if you’re interested.