It seems like in this case the FBI wants to sift through the phone for leads into other potential terrorists, and they want to establish the precedent that no manufacturer should be allowed to add true unbreakable encryption to some device in the future, encryption that would preclude even what they are asking of Apple now.
Whether they have a right to that information in this particular situation, under current laws, even for the purposes of preventing terrorism, is a matter for the courts (probably the Supreme Court) to decide.
Whether Americans have a right to use personal strong encryption will have to be legislated by Congress, not by the bluster and propaganda of the FBI, Apple, and the tech industry.