Why do they put sulphites in vinegar when vinegar is a preservative?
Fermentation naturally causes sulfites, sulfites are a by-product of the yeast activity . Sugars, particularly beet sugar is often bleached with sulfites. Even common table salt (iodized) contains sulfites, the list goes on and on. 

Per the Wiki Article: "In the U.S., labeling regulations do not require products to indicate the presence of sulfites in foods unless it is added specifically as a preservative; however, many companies voluntarily label sulfite-containing foods. Sulfites used in food processing, but not specifically added as a preservative, are only required to be listed if there are more than 10 parts per million (ppm) in the finished product."

It is not hard to find sulfite-free vinegar, it will often be labeled organic.