Well, I get where you are coming from with the whole "do anything for justice" mentality and I would agree with you that justice is important in any case whether it be a terrorist attack or a murder.
I think what Apple is trying to say here is that all their customers are people, real people with personal data that is theirs and theirs alone. Apple has worked hard over the years to keep their customer's data safe by introducing great encryption measures and security features to keep data from falling into the wrong hands. I almost feel like this falls under the same guidelines as "freedom of speech" in a sense that if I ever saw someone shouting racist remarks in public I would think that person is down right awful and has no place in modern society, but at the same time I would fight for the right of every person to say whatever the hell they want on public ground. In turn, Apple is fighting for the right of privacy and security no mater what the case or person. your data is privet and belongs to you. Also, the FBI is literally ordering Apple to comply to a request that is unjust given the situation.
This would not be just a "one time thing" once a backdoor, weakness, exploit, or any other form of tampering is created then that would open up a whole new world for other departments/people to take advantage of it. Security is important whether you are watching stupid cat videos on your phone or you are a CIA agent with classified documents. It would be a nightmare if someone out there had the power to backdoor into your phone whenever they wanted to... Hello NSA!!!