Otherwise it sets a precedent and could potentially lead to “I don’t want to play against forts/events in this game. Let’s roll off”.
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. Ex:
You can also choose to play them as regular islands with no special effects.
Mysterious islands chosen as home islands might show game text when placed with the face up. Before starting your game, choose whether or not you want to use that island’s special effects.