The existence of the child porn might be a forgone conclusion, but there is a lot of other stuff on the hard drive. And that other stuff is not a forgone conclusion. So by decrypting the hard drive, he would be giving them information that is not a forgone conclusion.
I believe the persecution would argue that any newly discovered evidence can simply be excluded by the court. The legal system's philosophy is that it should be all-powerful, and that it will parcel out what it deems to be our rights.
Of course, Free people know that rights are obtained and maintained by individuals themselves - eg the second amendment.
