It's just oddly different from everything else in the game. When you cash rush a unit in a city, it doesn't just stay inactive (which it is), you need to have the city empty. One unit per tile means one unit per tile. I don't see why airlifting would change this mechanic when nothing else does.
That is so unbelievably dumb and illogical. It defies the very principles of progress and improvement. Can you imagine what would've happened to human civilization if through history people had adherred to the philosophy of "Why change anything?"
Nothing, that's what; because people would never have invented agriculture, and continued to be nomadic hunter-gatherers, because
why change?
Just because presently Civ5 is designed so that a city must be empty to purchase a Land unit, doesn't mean that it should remain that way forever. Anyone can recognize that it is an unnecessary inconvenience; and IMO it's evidence of sloppy game design. It would take little to no effort to modify the game mechanics to give purchased units a one-turn exception to the 1UpT rule -- but
apparently that's asking too much of Firaxis. After all, why should a
game be more convenient?
Perhaps for the sake of making Firaxis' programmers' jobs as easy as possible, why not discard the mouse and GUI entirely, and instead require the player to
type all commands in programming language.
Contrary to what certain players might think, it's
not our responsibility to make our ideas and suggestions as easy as possible for Firaxis to implement; the goal is to improve the game. When purchasing units, the 1UpT rule is an needless frustration and should be modified to make purchasing units more convenient.
Returning to airlifting: A unit should be allowed to be airlifted to a city even if another unit is occupying that tile. The airlifted unit will be inactive for the remainder of the turn after it's been airlifted, and the next turn one of the units will have to be moved from the tile -- no different than if a new unit was produced in the city.
It's simple, logical, balanced, and convenient.
That's how a destination city would function without an Airport. If the destination city had an Airport,
multiple units could be airlifted there in the same turn, and each unit would be inactive and stack until the next turn.