It is that follow on that is the back door. So now Apple is in the position where they have to unlock phones on demand, and not doing so will subject them to fines and imprisonment (you cannot easily appeal an order to do something in a case which it is clear you have the capability to do).
What is worse, if they are forced to comply and develop this capability, and then design the next generation of phone so that they once again don't have a capability to break into it, that case then is even harder. The FBI can sue Apple for intentionally obstructing justice.
Don't be fooled, this isn't a case about one phone and one set of people who have shot a bunch innocent people. This is very much a calculated strategy on the Justice Department's behalf to get back doors into phones and create legal precedents for which they can compel those back doors.
That will force the group that doesn't want back doors to get Congress to create a law that specfically forbids them. And the Justice Dept knows that would be very hard for folks to achieve.