If the FBI is able to force Apple into making a new version of iOS just so the authorities can break into a phone, Apple is concerned about what else the FBI could request. Speaking with Univision, Apple executive Eddy Cue brought up other possible requests that'd be easier to justify if the FBI wins in the San Bernardino case.
Cue, repeating an issue Apple has mentioned before, said the FBI could eventually request Apple make software that lets the authorities remotely monitor a phone's camera or microphone.

This problem builds upon the idea that an FBI win would set a dangerous precedent. Not only would it allow the authorities to request many phones be unlocked via special software, it'd make it easy to request software be installed for the purpose of surveillance.
source:nextpowerup