> Whitening using any non trivial encryption key seems weird to me.

It's because there was an era when drives were expected to be able to do 'hardware' encryption with a user provided key, so reusing that hardware to also do whitening even if the user didn't provide a key was very convenient.

Plus you get all the other benefits - ie. a single scsi command can 'secure erase' the whole disk in milliseconds by simply changing the stored key.