Setting Precedent
There's some interesting bits on this issue. John Oliver sums it up nicely.
I've always wondered how badly the FBI really needs Apple to build the requested super-key. Setting aside how bad it would be if such a key got into the wild–it would–why doesn't the FBI just go to the NSA? Former national security official Richard Clarke has the same question:
“Every expert I know believes that NSA could crack this phone. They want the precedent that the government can compel a computer device manufacturer to allow the government in.”