[vlovich123]

Human systems (contracts/legal) step in when technological systems can’t. Until there’s homomorphic encryption, enforcing that you’re using their key can give them piece of mind that they can revoke it. While true that you could be doing anything with that data, a contract in good standing and normal human ethics probably adds a high degree of likelihood that you’re not. However, if the relationship sours, they want the freedom to revoke quickly without needing your good will. If your backups aren’t using their keys, I think you’d be violating the contract.

But yes, from a purely technological perspective security theater. They could also misunderstand what’s happening and it’s also not worth it to try to explain for you at the risk of losing the contract.

[X-Istence]

I don't know how this is for others, but in the environments I am in we use a different KMS key for our backups so that if something were to happen, like a mis-click in a web interface or an accidental terraform destroy, we can recover the data.

It is also stored in a different location than the original (different AWS account).