There is a big gap between ‘supposed to’ and the real world. Pick up any bottled dressing at a typical grocery store, and sugar or corn syrup will likely be a top ingredient. See the totally unnecessary addition to ranch dressing, for example:

http://www.kraftrecipes.com/products/kraft-16-fo-dressing-li...

Or even if you are making your own, a surprising number have sugar or honey as ingredients.

https://www.tasteofhome.com/recipes/italian-vinaigrette