Ensure that the original 4:3 framing is left intact and have this selected by default. The 4:3 area will be remasted to 1080p.
The selective zooming can be an option for people who would prefer to fill the screen, rather than simply zooming into the centre of the 4:3 area the zoom is carefully placed on each shot to ensure nothing important is cut off and it still looks ok.
Maybe this can be done with blu-rays?! I have no idea.