Why does an organic salad dressing contain Xanthan gum.?

Xanthan Gum is an organic product. It's a derivative of a bacteria that occurs on leafy vegetables.

Probably because it is derived from organic sources. Don't assume that "organic" means "all natural'. Processed food is never natural (You don't see bottles of salad dressing growing on the dressing tree.). Organic simply refers to the farming techinque, as far as I know; organic means no artifical fertlizer was used to grow the crops, and no pesticides or funguicides were used during growth, as far as I know.

According to "wikipedia", Xanthan gum is a "natural gum polysaccharide" which is produced by a "process involving fermentation of glucose or sucrose by the Xanthomonas campestris bacterium".

Organic bacteria acting on sugar derived from an organic source equals organic food additives. ^_^

I think you are being disingenuous. You seem to be creating your own definition of "organic."