Game Theory: Why certain Principles for certain Backgrounds & Archetypes?

The one area of Hero Character Creation I find myself stumbling around is the decision that certain Principles are only available if you pick a certain Background or Archetype. For example, if you have a Military background, you should have an Ideals Principle. Why not an Expertise or Responsibilities Principle? The idea that choosing Option A also locks you into Option B seems to run counter to everything else about this game, with its free form Build It How You Want it mentality.

Any issues of game balance that I am overlooking here?

You’re free to choose ones which you feel might make more thematic sense for a character you’re building.   I see that piece as moreso narrowing your focus instead of looking at everything for a guided method.  

Honestly it is the only retcon option I look at and say,” Yeah if you don’t like your choices provided here go ahead and take what you do want. This won’t take up your retcon.” Everything else extra power, increase red zone die size, extra red ability, and to a lesser extent dice swapping at least make sense that they are mechanically major enough to warrant the use of that part of character creation.

Using it on a different principle just seems a bit wasteful.

This. I’ve basically told all my players that they should treat the categories as suggestions, especially when they get the same one twice.

I see it similar, but it would be nice if there were 2 to pick one from rather than a singleton…

Yeah, I've found myself choosing backgrounds based mostly on which ones give me access to a more fitting principle.

Here's an idea: When using the guided method, you may pick a principle from the categories provided by any backgroud/archetype that you rolled, regardless of which one you actually picked. Not sure yet how to adapt that to the constructed method, though.