I love Louis Rossman's video about why he does not use Apple products. Near the beginning, he talks about how he worked in a recording studio, and people would bring in background music they wanted to sing along with on MP3 players, and he was used to just copying files off. Which worked just fine until he encountered his first iPod:
The other mp3 players didn’t get MPAA’s approval because they allowed copying music, ie “they were tools for piracy”.
Apple’s system was genius: It could only synchronize with the system it came from, making it a peripheral of the computer, not a node in a network. This restriction lived on for the iPhone, making Apple fans curse and Android fans laugh, when a computer was required to install an iPhone, for years and years after it wasn’t technically necessary. But binding with 1 (one) computer was the agreement if Apple wanted to host music under benevolence from the labels.
Apple would have never been able to get agreements with music labels if the iPod had been open. And thus, it lived on, while all other mp3-player brands died.
It is a consequence of the music industry and an emergent property of IP laws, not a standalone decision from Apple, as opposed to making all their products closed-source, which they don’t have to.
FWIW, you can trivially pirate music from an iTunes library [1] with an external hard drive or USB stick or, in fact, an iPod mounted as an external drive. You could also play music from an iPod that was plugged in, IIRC, which should have been enough for the recording studio/karaoke use case. But you couldn't just copy music from an iPod to your own library, indeed (I doubt the music industry would've gone along with that for very long, either).
[1] except DRM'd music, such as music purchased on the iTunes Store between its debut in 2003 and 2009.
https://youtu.be/sfrYOWlKJ_g?t=163