In two words….playability and stalling…
Now imagine US spend 400 IPC on battleships, carriers, destroyers, trannies with men and guns, etc etc and then sail this huge fleet to France in order to make an amphibious assault. Now Germany spend 6 IPC on a lone Sub with the ability to stall this huge invasion fleet, and there are no ways to deal with it. I guess you would have complained in this case too, asking why a lone Sub can stall this huge fleet, and if there were any cases in the real war to justify this. So, since the designer didn’t want this game to be too gamey and tricky, he made the rule Wittmann is talking about, a Sub or Tranny don’t make the seazone hostile. The Tranny is unarmed and the Sub is hiding under water, or submerged as the experts say. Now Germany need to spend 8 IPC on a Destroyer if they want to stall US, as if one single Destroyer could possible stall 200 Battleships in the real world, but as a Destroyer that can be easily removed by two UK planes before the US attack, and everybody is happy.
But do it make sense, is it rational ? You tell me. Look up the D\day landings and Battle of Normandy, and tell me how many ships were sunk by German subs during that campaign. I guess that will give you an answer.