If that's the case, why do they even need the drive to be decrypted? All they'd have to prove is that the drive was not owned by anyone other than the accused a

If that's the case, why do they even need the drive to be decrypted? All they'd have to prove is that the drive was not owned by anyone other than the accused and could then say that there was CP saved onto this drive by the guy. If they matched the hashes to data on the drive, the files are already on there unencrypted. Could be some space left by a now-deleted unencrypted partition, or maybe some leftover data in a temporary location where the data is saved before being encrypted. Together with the witness testimony, that seems pretty compelling.