Apple can't simply register devices into your account, they would have to have access to your iCloud account, which they can only get using one of those keys.
I guess if the devices are only willing to share keys with other devices which can individually prove they have the iCloud password. I wonder if it's an online procedure or if they just upload the keys to iCloud, encrypted with the user's iCloud password.
When registering a new device you have to authorise it using one of your existing devices.