An awesome idea from Apple. Being an organ donor costs you almost noting - you don't need that body after you're gone - and it can save so many lives. Why wouldn't you be one?
Apple is hopefully tapping into a big audience of people who just haven't signed up because they haven't found the time or forget, rather than those who actively don't want to be on it.
You can guarantee some people will have whole lives ahead of them that may not have had otherwise, purely because of this move.