Makes sense to me, who could be bothered to clone drives and then have to wait for them to be re-encrypted per device. Instead of just modifying the firmware once?

Especially considering the devices would be sold to strangers and it's quite unlikely the harddrive would make it into the hands of someone without access to the paired system.

I suppose the re-encrypting could just be a second key, but still would require processing that is not worth anyone's time.