OK I found that I can click the Rectify tab on other people's maps.
Maybe at the top of the page it should say: You are rectifying somebody else's map.
any changes you make will change that person's map forever. Be careful. Or
any changes you make will be saved in a new copy in your personal map list. Or
ny changes you make will not be saved and will revert upon closing the browser. To continue to edit this map, press the Copy Map button in the upper left corner of page XYZ...
OK I found that I can click the Rectify tab on other people's maps.
Maybe at the top of the page it should say: You are rectifying somebody else's map.
Anyway the program needs to say something!