Allied ftrs CAN defend, but they CANNOT attack.
A typical scerio is a UK 1 AC buy with the US landing 2 ftrs on the AC on US 1.
This Navy is all but untouchable for the Germans b/c the US ftrs defend with the Brits.
However, moving the UK AC on UK 2 to attack say German ships off Spain, the US planes can indeed be taken as loses but not fight offensively in the battle.
It is no different then land combat,
multinational defense - yes,
multinational attack - no.