The issue is the precedent that is made.  Once they create the special firmware, even if it is locked to one device, they have stated that law enforcement have already told them they have hundreds more devices they want unlocked.  They would have a much easier time going to court and compelling Apple to  resign the existing special firmware they made to work on these other hundreds of devices. Apple could no longer argue they had to create something to fulfill the warrant, since it was already created.