If they really wanted that data, they could get it with their current capabilities. They don't really want that data; they want the legal precedent to compel companies to subvert their own security mechanisms, and they want to intimidate one of their harshest critics (Cook). That's part of their broader strategy; if we can compel you to break security you built, you build security you can't break. The next logical question is whether they can compel you to not build security mechanisms you cannot break in the first place. That's the legal question the FBI really wants to ask.