One of the problems with the tipping culture here in the US, is that the tip comes at the end of the meal, therefore patrons sometimes get prejudged on what the waiter thinks they may tip. I have had friends (who were waiters) state that if a table orders a round of ice water, they assume they will not get a good tip as the table is cheap. There have been cases where certain minority races will not get very good service, as the wait staff assumes they will not be getting a good tip. If you are a visitor to the US, and are in a tourist area, be sure to check your bill and ensure there is no tip already added. This happens to my Uncle frequently when he is in Florida from the UK.
If an establishment adds gratuity to my bill, that is exactly what the gratuity will be, not a penny more, which usually costs the server money.