If the FBI wanted to be "extremely reasonable"
They would have acceded to Apple's request to file this case under seal. The important factors to consider are why did Apple make that request, and why did the FBI not honor it? Here are the possibilities:
1) Apple asked for it because they were going to do what the FBI asked but didn't want the bad publicity of helping hack into an iPhone. If so, the only reason for the FBI to not honor the request is because they wanted this case to set a precedent.
2) Apple asked for it because they were going to fight it and didn't want the bad publicity of "Apple refuses to help stop terrorism". If so, the only reason for the FBI to not honor it is because they wanted to drag Apple publicly through the mud, hoping public pressure would force them to comply, AND probably also to set a precedent.
Neither is the action of an FBI that wants to be "extremely reasonable".
This case has the perfect optics to put maximum pressure on Apple. Dead domestic terrorist who is known to be guilty and committed recent act on American soil fresh in people's memory, a phone owned by a government body, and its an iPhone 5c which is easier to bypass in this manner than the 5S/6/6S which may not be possible to bypass in this way due to the secure enclave. Exactly the sort of case you'd choose if you wanted to set a precedent, because it has the maximum chance of success in court.
Only a fool would believe this case wouldn't set a precedent - whether a court orders Apple to comply or they comply willingly, the next time the FBI wants the same task done for an iPhone Apple is put into the position of arguing "this time its different". The question becomes not "should they help" but "where's the dividing line between where help should or shouldn't be provided". What's the public reaction going to be if they help in this case, but refuse to help in the case of a terror suspect, or child molester suspect? Even in a case of "simple" murder the cops could get the tearful victim's sister on TV saying "why won't Apple help when it might provide my brother in law did it / is innocent, when they have provided this exact same help in other cases?"
NYC alone reported over 100 cases where they have an iPhone they can't access. Police are going to be lining up at Apple's door with thousands of requests. Can they really deliver a special version of iOS for each one? No, they will have to have a special "hack" version of iOS they install on these phones. Think that will never leak, or Apple's corporate servers are hacker proof?