It seems like it is definitional: Fe users have a basic assumption that people directly cooperate on feeling right. Even INTP's show this. (If you look hard.) Fi users assume people indirectly cooperate on feeling questions.
What's the difference?
I don't know. It seems to be quite hard to express without just saying the feeling focus is different.
Both sides can approximate the other side's practice, but it isn't their preference. And if it's not their preferences, maybe especially Fi users will "feel wrong" and react.
How would an Fi user encourage an Fe user to approximate Fi practice? By making them fail, I guess, making them feel bad about themselves.
That actually seems to be what Fe and Fi users encourage each other to do: to fail. To not perform according to the standard set out by the other side.
And quite possibly T types are loudest in complaint because their F side is (at least theoretically) much more reactive than active.