There's not a single customer I've ever run across who's going to halt a contract because you can't purge their data from your backups fast enough. They're signing up because of what you offer, not the termination clause.
The fact that you own the KMS key doesn't stop me from reading/writing that EBS volume. It doesn't offer any additional security guarantees, especially if I was already encrypting the EBS volumes with a KMS key.
If I am a SaaS offering, whether I use a KMS key I own or one you own doesn't change the fact that I still have access to all of the unencrypted data that is silently being encrypted by those KMS keys, I've got access to the layer above it.
Sure, if the contract ended they could revoke the KMS key and now the data on the EBS volumes is no longer readable by me, but any backups I have of that data is still within my purview.
If there was a very deep integration, essentially column-level, it could make a lot more sense, but that could give quite a big performance & cost hit as well as limit what you can do (essentially you wouldn't be able to do any matching on content level).
Other examples of such meaningless-and-impotent controls in infosec include "must run a firewall on your Linux server on a private subnet", and any policy requiring password complexity/rotation. But as a business, it's more productive for us to just tick the checkboxes than to spend resources on educating the market about the actual best practice (unless you're in the business of education).
I don't want to imply this is common - it's not. Such cases come from the "arbitrary enterprise rules" side rather than the "SaaS/tech" side.
