Giving it away means there's two copies owned by different people. That's why you have to delete your backups.
Destroying it, accidentally or otherwise, means there's still one copy owned by whoever bought it. That's why you get to keep your backups. What the fuck else do you think a backup is?
A backup is not treated as meaningfully different from the original copy. That's why you're allowed to make one, at all. That's also why you can keep the backup - you still own that copy. Even if you've transferred formats. Which is also explicitly allowed.