Personally, I don’t see a logical reason why fighters can only scramble from an island. I figure the rule is probably the result of playtesting, and that allowing fighters to scramble from, say, Norway, would somehow unbalance the game. But it seems like it would benefit both sides equally overall, considering the size of the map and the number of territories on it.
Perhaps it was made that way for the sole purpose of increasing the value of islands with no IPC value.