You answered your own question. They allow this because often members of a group want to share their headphones without fully setting up a "Family Sharing" acco

You answered your own question. They allow this because often members of a group want to share their headphones without fully setting up a "Family Sharing" account. You can only make changes to your "Family Sharing" account once a year due to demands from the music industry. They were worried about the revenue loss from friend groups registering as families and required that it be difficult to form a family group. Everything about sharing Apple devices between family members is awkward because the RIAA dictated the structure of Apple IDs when they were first being designed. Let alone my Apple watch that can only pair with one of my phones at a time...