My understanding is not that they have good evidence that there is CP on the encrypted drive, but rather that they have good evidence that the drive belongs to

My understanding is not that they have good evidence that there is CP on the encrypted drive, but rather that they have good evidence that the drive belongs to the guy and that he can decrypt it/has access to/knowledge of its contents. The idea about not having to give up a combination/password is that by doing so you would essentially be admitting ownership/access/knowledge about the contents that were protected by that combination/password.