iMessage isn’t meaningfully end to end encrypted either, so it shouldn’t get a different color by that logic.
The automated backups (which are not e2ee) serve as endpoint key escrow (to the middle service), backdooring the e2ee of iMessage.
i am surprised (am i really?) that apple never implemented a toggle to E2EE your icloud backup, just like you can backing up locally through itunes. thankfully you can just completely disable iCloud Backup.
iCloud Backups can be E2EE encrypted if you enable advanced data protection.