I would say because with control, you are "flying in a NFZ"And that very much coincides with what the OP said, "i pulled up the throttle all the time and tried to get it back but it kept just going down." I understand they have coded the software/firmware like that but why not give the pilot the option to fly manually to a safe landing position if there is water underneath? Definitely an oversight on DJI's part.
It is the legality of the thing that DJI is trying to prevent?