Informal
institutions (i.e. social norms) are in the domain of Ti (and to some extent, Fe), not Fi. Fi is much more individualistic in attitude. It does not so much evaluate other people's behavior from a framework of social norms, but from an understanding of personal character. E.g. someone might make a joke that goes against a social norm, but it is not being held against the person because it is understood that the joke did not originate in bad intention or ignorance. Such are the evaluations Fi makes.
That being said, we can also conclude what the problem is in LSE's in this example: an inability to make Fi judgements, which leaves them no other means than to assess other people's actions from the perspective of Te or Ti. It is indeed not uncommon for LSE's to stick to conventions they've acquired in the past, and keep on judging things according to those conventions.
Note that Fi-
seeking does not truly mean that one is actively seeking Fi. We are never aware that we are lacking our seeking function or are in need of it, until we come under its spell.
Ergo, what you are describing here, does not originate in your Fi-base, but your Ti-role. If we assume, for the sake of discussion, that you indeed are EII. Your conclusions are right, your interpretation of the reasons why is not.
I've always thought it amazing how Americans are capable of eating stuff with just a fork, without the assistance of a knife. But somehow I also had the impression that this form of etiquette was nothing else than institutionalized behavior originating in boorish European immigrants.