No, it is not illegal to not tip--tipping is voluntary, except as  set forth below.The only time a tip is mandatory is if the restaurant's menu or other prominent material (such as a notice on the table; or a prominent notice on the window; etc.) states clearly that in certain cases, a gratuity or service charge is added--this is fairly commonly done for large parties (e.g. 6 or more diners). If notice is provided prior to you ordering your meal that you would owe a service charge or gratuity in certain circumcumstances and you go ahead and order, you'd have to pay the gratuity. However, without prior notice, you would not be obligated to pay.