Let’s clear up this confusion of yours.
You could do it differently, but usually we rotate the spring arm and not the camera. So it’s not directly rotated but is still affected by it’s parent’s rotation, and if we get the camera’s rotation it will still give the result in world space.
How do we rotate the spring arm though? We also don’t do it directly.
Ackshully
Of course there is a direct manipulation in the background 
but we’re talking about the interface here
We rather rotate the controller with mouse inputs, and the spring arm gets affected by it when we enable it to use pawn control rotation. So that phrase of yours “the control rotation knows [enter component]'s location” is kinda wrong, it doesn’t “know” anything, it just affects them. So since we’re already rotating the controller ourselves, it’s only natural that we get the controller’s rotation when we get the controller’s rotation 
And in this case the camera will be facing to the same direction with the controller because the spring arm’s rotation is determined by the controller’s rotation this way (again, because use pawn control rotation is enabled for it), and the camera also rotates with the spring arms because it’s a child, and we’ve set it’s initial direction parallel to the spring arm ourselves from the viewport.