We have hard water to downstairs sink tap/ outside and the dishwasher ~ the instructions with the Bosch dishwasher stated that it wanted hard water :-(
So what happens in an area where the water is naturally soft? Maybe they mean don't feed ion-exchanged softened water.
Our Bosch dishwasher seems happy enough (it's on a hot feed, which is softened), although given dishwasher usually take salt to soften the water anyway i can't see why it would make any difference.