Remember that the game is still in pre-alpha. Issues like this will likely be fixed before the final release.
The cities can't have subway because they are to small but can have international airport in the Great Works
I don't understand Maxis
The two are completely different. Subways have to be within the city, which is too small for them. Airports, however, are built by everyone in the region, outside the cities. They help everybody in the region.
Of course, I know the game system, but what I mean is that if a city in real life is small and therefore has no subway, rarely has international airport.