Because the soon to be sunk merchant ship was radioing the subs position to the allies the whole time the sub was standing by awaiting for the ship to get it's passengers safely off. The sub was spending TOO MUCH TIME on the surface exposed to enemy hunter killer teams. German subs were being sunk and experienced skippers (captains) and their men were being killed because they were "being kind" (also known as "too nice"). If "ships to be sunk" did not radio off the sub's position, and if enemy hunter killer teams didn't destroy so many German Subs/Skippers/and crewmen; then the "Restricted" warfare would have cost the Germans nothing, and they would have and could have...continued.
World War 1.
WWI; Guerre de Course-Merchant Raiding aka Commerce raiding.
"Guerre de Course", Merchant Ship sinking.
IJN subs were deployed for "fleet warfare", not merchant raiding. The costly job of sinking allied warships fell to the IJN sub fleet. It's estimated that IJN submarines sank less than 50 merchant ships during the war.
Ships try to avoid sinking, submarines sink on purpose...
By sinking them!
Sinking ships; exploring the depths.
France and England
There was no formal pledge not to sink civilian ships, however the concept of neutrality was accepted and agreed by all the western powers. The illusion of neutrality was shattered when a German submarines sank the unarmed passenger liner Lusitania.
As an ingenious new method of sinking warships.
In the WW1 the US declared war on Germany after the sinking of seven US merchant ships by German submarines and the publication of the Zimmerman Telegram. In the WW2 was Hitler to declare War on the USA on Dec.11,1941.
Likely Churchill.