It is in Apple's best interest to find out what the exploit is, how long it's been out there, and fix it instead of playing legal games. Such an exploit in the wild is a security concern for every person who owns that phone, not just criminals. There is no telling how many people are out there accessing privileged information in the private and government sectors with that phone. It is a concern covering criminal activity all the way to foreign government spying.

As a bonus, if it's an exploit that's been known from before the FBI's claim that only Apple can help them breach the phone, then we know the FBI was full of it. The FBI has been known to use questionable tactics before and backed out of cases that weren't going their way to avoid revealing those questionable tactics. This whole iPhone thing reeks of this.