The FBI isn't asking for Apple to give them the update to apply themselves. They're asking for Apple to apply the update. To brute force another phone, the FBI will have to go back to Apple.
The only restriction on this law that Apple could claim (aside from legally out-there options like equating writing software to forced speech) is "unreasonable burden," but this is something that almost any of us could hack up in a day given access to Apple's release build system.
In the future, Apple could make a phone that is actually secure. For example, requiring the phone to be unlocked before updating firmware.