It's easy to see why you wouldn't want someone to decrypt your laptop. What about Bernie Madoff? Wouldn't you be furious if all data produced by a corrupt hedge fund manager was encrypted? Wouldn't you want the DOJ to be able to hold them in contempt of court? If you hamstring your government, you aren't allowed to complain that it is ineffective.
"The trouble with fighting for human freedom is that one spends most of one's time defending scoundrels. For it is against scoundrels that oppressive laws are first aimed, and oppression must be stopped at the beginning if it is to be stopped at all." -- H.L.Mencken
There's a line, and past that line, law enforcement has no power to enforce laws at all. Removing the DOJ's ability to collect evidence is, in some cases, the equivalent to denying a detective access to a crime scene. If all productions of electronic evidence can be encrypted, and the defense is not forced to decrypt those files, then dirty corporations only need to encrypt everything to defend themselves in court.
Defense: "We produced everything you asked for."
Prosecution: "We can't even verify that statement. Decrypt it"
Defense: "We don't have to."
Case closed.