[fncypants]

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.

[shalmanese]

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.

[anotherman554]

Copyright law does not require royalties to anyone. People can negotiate for royalties or not negotiate for royalties.