This issue does come up from time to time and it is something we investigated for possible amendment, however if we truly went with a rule that only allowed valid swapping with the right AP, which includes taking into account ZoC penalties, it would severely limit swapping in game in general.
That and the potential difficulty in conveying to players exactly why some units could swap and why some other units could not.
Sure, we could just indicate a unit couldn't swap, but we suspect players would either think there is a bug (e.g. because some could swap and others couldn't) or players would specifically want to know why, with some sort of in game feedback, indicating this.
There is that potential for in game frustration when things either feel like they are not working as they should, or it is not clear enough why things are restricted. Especially when looking at the WWI map, there would be many cases where units simply would not be able to swap along a frontline due to AP and ZoC restrictions. Swapping would then have the potential of becoming a non thing in game.
So for now, we've just left things as is, e.g. where swapping is a simple enough mechanism with limited restrictions and the only penalty is the unit morale hit that occurs.