I would have to agree with Autarch on this one.
The coasts of the USA and Australia were vast when compared to some of the tiny pacific islands. If you look at the aerial photo’s of places like Iwo Jima and Tarawa the entire Island was basically the airfield and AA and other defenses. The Idea of the scramble off the Island is to allow airpower to protect your fleet when it is cruising off the coast of the Island and serves as a carrier that can land unlimited aircraft as well as bombers (although they don’t scramble). The extra range bonus given by the airfield also makes the aircraft’s movement operate as if they were on a carrier in the surrounding sea zone. The airfield also helps the defender to keep the island by making it more difficult to destroy the surrounding and defending naval fleet for the invader before they can land any units to attack the Island. This is also advantageous because you can now park a fleet outside of your island airbases and reinforce it with fighters and tac bombers much faster than ferrying infantry to it by transport.
I think that perhaps it would have been better to have created maybe two types of units, the airdrome and the airbase. The airdrome would operate exactly as the current airbase does in AA PAC 1940 giving the scramble ability on Islands only as well as the +1 movement bonus. The new “airbase” would not allow scrambling but would give a +2 to aircraft range if they flew two and from an airbase or airdrome, using the airbase either upon launching or landing. This airbase would be more useful in light of the inability for scrambling on non-island territories, especially in the upcoming Europe with UK, Moscow, and Germany. I would guess it could be developed into a house rule with Airdromes being cheaper, 12 IPC’s I guess and Airbases remaining 15.