The salt content of salted butter can vary from brand to brand, and from region to region. One batch of butter may have 1.5 percent salt and another twice as much. Even if you stick with one brand and know its exact salt content, it is a headache to calculate the amount of salt you must subtract from a recipe that calls for a given quantity of sweet (unsalted) butter. Since the recipes from virtually all serious cookbooks are based on the use of sweet butter, people who cook with salted butter create an extra complication for themselves (unless, of course, one's religious code dictates its use).
** Asian Recipes