Lavabit intentionally structured their service such that there were only two ways to get at a criminal's email: obtain a copy of the suspect's private key or compromise the entire service. Lavabit was capable of reading the suspect's email, but only by slurping from the firehouse and reading everyone's email.
What Lavabit set up to be their greatest strength ("Nobody can read your email unless they somehow have our SSL key") turned into a terrible systemic weakness ("If the government want to exercise its legal right to obtain a warrant and read a suspect's email, then the only way it can do that is to get the key to everyone's email").
My personal real-world analogy is a safe deposit box that opens either with a key the suspect has or with a master key that opens everyone's lockbox. The bank would defintely be required to turn over the master key. The fact that the only key that opens the suspect's lockbox also opens every other customer's lockbox is a fault of the bank's own devising.
(Of course, who the suspect is in the investigation is irrelevant to everything else.)