If it was invadable, the axis would take it on I1 to deny it being used as a base for Uk fighters. To model how it effected the Italian situation it should be un-assailable or the benefit would never be realized.
If UK was allowed to post a fighter at start, all that would happen is the German forces would wipe out the defending fighter with landing 2 infantry plus planes. If UK got extra pieces on setup, then Germany would have to get some compensation which would upset the set up elsewhere. The result would be extra german pieces and dead UK fighter, which is a net gain for Germany.
I playtested Malta in my map phase 2 and i feel the KISS game theory would just make it not invadable to make it effect the game as small as possible, but allowing some flavor to be added for UK. BTW the same would not apply to Gibraltar.