I always use plain old cold water from the outside tap to clean my car and BMW is always nice and shiny - no problems there. I take it you wash your car in warm pure water...with a dash of vision?!
I like my car 'detailing' and in my area the tap tds is 050. When I rinse the car down it is still shiny, but covered in light water spots. Until I dry it off and Aqua-wax it.
Or sometimes I just rinse it off with pure (which does have vision in) and the difference between a tap wash and a pure rinse is huge.
I take it you are in a very soft water area?