Restaurants in Miami Beach do this frequently because they get a lot of visitors who don't tip - having said that, you can ask the manager to reduce or remove the tip if you feel the service didn't warrant 18% (I can only think of ONE restaurant in Miami Beach where 18% would be appropriate) - and if it was not disclosed on the menu ahead of time, you are under no legal obligation to pay it.

Since you mentioned Miami Beach, I want to remind folks to be wary of a scam that many restaurants in that area like to pull - they either add additional items to your bill, modify the prices or sometimes substitute a "shill" bill (someone else's bill with the same number of entrees but higher prices or a dessert/appetizer added).

Always check your bill carefully when eating in Miami Beach (this probably holds true for most tourist destinations worldwide) and don't hesitate to threaten to call the police if they make a stink over the wrong bill (insisting you ordered something you did not) or refusing to reduce the tip amount.