Hmm I'm confused, so is FBI asking to unlock just this terrorist's phone or setup a backdoor to all iPhones? If they are asking to unlock only this terrorists phone I don't see why Apple should be fighting a case over this.
The FBI is asking Apple to build a tool to backdoor this particular iPhone. Once the tool is built, it gets much easier for them to ask Apple to use it on other phones. "We'll only ask for it this one time, honest" isn't a believable promise - Apple winds up with a much less defensible position later, since they already have the tools necessary.
Think of it more like going to a company that builds safes. Not asking them to blow the door open, but invent some magical safe breaking tool (that has never existed) and would then allow them to open whatever safe they want.
Nevermind the fact that the point the safe company success relied on trust that you cannot break into their safes in any way.
