fair enough, you can turn it off, but it is supposed to do it and I've always found it very useful.
To set the mirrors to your desired angle. First adjust the right mirror, this does move them both but anyway. . .
adjust right mirror till it is in the correct position. then adjust the left. when adjusting the left, it adjusts the left ONLY, therefore leaving the right one in the desired postition whilst adjusting the left.
NOW
when you can reverse parking, just move the right mirror downwards, then BOTH move down so you can clearly see the gap/lines/curb that you are parking next to and you don't have to bother moving them both down individually . . .
then move it up, they both move back up and hey presto!!!
hope that helps