If you throw the transport in the attack, you’re throwing in the ability to take an extra hit… you’re also throwing in the German sub actually being able to fire and hit the transport, making the entire point a waste.
Ahh this explains much of your confusion.
If you attack a sub & transport with planes only, all that’s able to hit the planes is the transport.
Even if you attack with a ship though all that can be hit is the trannie. Why is this, because of rolling by columns. The rules actually state that the defender has to roll his lowest numbered dice first, in this case a transport. Should this ship hit, and you logically kill your transport then the sub cannot fire back as all that remains are planes.
Why not take the advantage of rendering a sub useless besides as cannon fodder, and why throw in something that allows them to actually fire at it? Yeah, you’re giving yourself twice the number of hits before your plane gets taken down… you’re also tripling your opponent’s hit rate until such time comes as the transport is lost.
Like I said if you observe the column rolling procedure this will not happen, but sending two planes against a trannie sub is very risky. I would assume that the trannie will get two shots and therefore has about ~30% chance of hitting a plane 1-(5/6 X 5/6). The chances of two transport shots both missing. This is frankly not how I would like to start a game on R1.