The difference between the british practices and the german is an equity question. The British were acting within the accepted laws on the carriage of contraband, the rights of a belligerent to stop and search, and if necessary apprehend any vessel entering the declared war zones. The Germans did not. For them, any ship, friendly, neutral or enemy was fair game. Later on the high seas they adopted a policy of shoot on sight, no stop and search
Heh, not to defend the nazis but, for the Germans the ship of legality had sailed in WW1, after the British broke international law and began to intercept and divert merchants and seize neutral cargoes there was really no point in unilaterally abiding by the rules. Why would they expect different in WW2?