They require a promo? I was under the impression that it was free. If said promo costs sufficient gold or is far enough down the tree, I guess I can accept this.
Uhu.
http://well-of-souls.com/civ/civ5_promotions.html
I don't think it costs gold, but experience points. If you want to have a transport unit, you must waste one of your promotions on embarkation instead of actual combat promotions.
So it won't be like all your enemies will flee by transforming into transports just like that. And I think that's nice!
Off-note: Civilization series certainly isn't aiming for realism. Its developers want it to be a good game instead of a history simulator. Therefore people who are looking for realism from Civ are a bit silly, no offence!