This is about setting precedent. If Apple caves then the FBI can go to any internet device manufacturer and ask for a backdoor. It's getting to hard for them to go to each company one at a time. So they're trying to shotgun it and get access to the world of all internet devices (all smartphones, computers, cars and "the Internet of Things")
I have a friend who was approached by the NSA. He was a Sr VP and they came to him and asked for a backdoor. He arranged it, because it made sense. But there are many companies where it doesn't make sense and to protect their use base they won't just give one out.
So, you trust the FBI enough to just give them access to everything? And you trust them enough that they will only use it on special occasions. I don't
People have gone to jail for not granting access:
Ralph Nachio of Quest CEO of Bitlocker
Everything a Judge issues a writ or warrant for. So long as they go through a Judge, I regard their search as legal.
At least until we reach the point where we cannot trust judges to issue valid warrants.