Eugenioso, I think the way AoD "fixes" this issue is by making amphibious operations dependent upon the supplies that a naval unit is carrying. A single transport carrying a single unit that sets off with full supplies probably has an effective threat range of something less than its maximum range (2,500 km) simply because supplies will run out and it will have to turn around and head back to port.
I'm not certain of this. I'm certain of the importance of the supplies, but not whether it is this feature that restricts the AI operation. Hopefully someone else can confirm or correct this.