If vinegar is a preservative, how come they put sulphites (also a preservative) in it?
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.
Sulfites will often be added during the wine making process. Mostly to kill bacteria before fermentation starts. The bacteria would impart odd flavors to the wine.
Vinegar is made of wine, so the added sulfites are still present and must be labeled.