Wouldn't that mean that the manufacturer still has the secret key? That would make you vulnerable to government agencies,... and it is not the case if you use truecrypt or dm-crypt/luks.
That would be preferable, and would have the same properties as keeping a non-passphrase-encrypted copy of your LUKS key elsewhere (possibly a central repository), but would reduce the usability of their software that can 'unlock whole fleets of drives'. And there's no guarantee that there isn't even a secret third unencryption method, companies love to have future options.