Yes, it's contractual because of copyright. If someone (and many parties in the process) created it back then, there is a web of copyrights that require royalties to the creators. Contracts would control who this obligation is owed to (if anyone) years later.
What? It has nothing to do with copyright. It's a provision negotiated by unions for the benefit of members which means you can't employ anyone in certain unions without a minimum royalty schedule.
Notably, live action productions covered under SAG-AFTRA and WGA get residuals while animated works under IATSE did not until very recently.
