Cathy -----The 15% is a TIP not Service charge. Many restaurants started adding the 15% because some travelers either left nothing or very little as to tips. I make it a practice to add to the 15% based on the quality of service rendered by the server. I add between 5-15% additional if the service is good to excellent.
Unfortunately it has become necessary for restaurants to add the 15% to their checks due to the patrons that do not tip at all or very little when none is added to their check.