The difference between those and MI’s is that forts/events never have their legality or usage in doubt from the standard rules – they’re always fair game. The rules make specific concessions on Mysterious Islands that create a sort of gray area, which makes me understand the dilemma a bit better
They’re not as clearly addressed in the rules as forts/events/etc., but are still prominent enough that any organized event needs to account for them one way or another. If there’s no mention of them, you have to assume they’re legal to use even if some players may not like them.