Sorry, you lost me. If the prosecution "already knows", then why does the drive need to be decrypted? It sounds like they already have the evidence.
And if they don't? Well, that's the definition of a "fishing expedition", isn't it?
Child pornography is special, because merely having "possession" of certain information is a crime. The law itself is bizarre, so you get bizarre results like this.
A conjecture is not the same as knowledge, "in the abstract" or otherwise.