1) It may start from right to left, but it traverses all of the way across. Thus the furthest left point turns everything to the right white.
2) I'm not sure to the answer of your second question. Post the original binary image as a standalone file so we can run it.
Why are you setting the values to 255? Why not just use logicals (0/1) since there appear to only be two colors:
result = logical(cumsum(original,2));
Everything after the first true to the right will be white (1, i.e. logical true) everything else, to the left of the first true will be left black (0, i.e. logical false)
