While you can argue it might be nice to have the option to keep the older backup, it’s also kind of a handy feature that I can restore to latest OS and not have to spend time installing updates myself, my data conveniently seeded into the latest software.

At any rate, iCloud is not “backup” in the traditional sense, it’s really “cloud sync” for user data. In that context it’s not really surprising that connecting a new device to iCloud and restoring pulls the newer OS version, given you never really backed up the OS data at any point.