He complied with previous warrants, right? And since he didn't use PFS, when he finally did hand over his key, it put all previous requests at risk. The problem was representing more security then he actually had.
The security provided was fully within the threat model presented, which never considered this type of attack.
He does comply for the record. He've just sent his key on printed paper in place of usb stick.
The key was printed using a font that was too small to be readable by any means.