I do see why they didn’t want to complicate the sub rules w/exceptions and what not, sub rules are tough enough. It would have been easy to add transports must be escorted with surface war ships through enemy sub infested waters or they are lost however. This would have pacified some of us.
Just as other ships may, submarines may also move out of a sea zone where they begin their turn with enemy ships, even destroyers. The destroyer doesn’t stop the sub’s movement, as the sub didn’t enter the sea zone - it started off there.