The FBI explicitly requested a firmware image that is bound to a specific device (by checking a unique ID -- read the court order please). That signed image would not be usable on other devices without going through Apple again.
Well, because an actual judge would be required to make such a decision, and in each case, the merits would be assessed separately. So Apple could say no, and a judge might agree with them "right, the government has no grounds here".
(IANAL, and IANEAA - I am not even an American).