This is my main gripe. You either have the evidence to put the defendant in jail or you don't. If you don't have enough evidence to support putting him in jail, you let him free.

Innocence until proven guilty. I don't want to live in a society where law is determined by emotions and personal bias, even if some criminals end up escaping justice.

Side note: I can't believe I feel the need to state this, but evidently it needs to be stated (from comments in this thread): these two drives where set up as part of a backup solution. They contain my personal data. They do not contain child pornography.

The ability to convict a particular case on a particular set of evidence is unknowable before trial, where the facts are decided by a jury. It would be irresponsible for a prosecutor to attempt a trial if they know stronger evidence is available.

The prosecution was able to convince a judge that there is encrypted child pornography on those drives. The judge can't force a jury to accept that (a ruling of fact), but (s)he can make a ruling of law that the defendant no longer has a fifth amendment defense to producing a decrypted version of those drives.