The tipping mystery solved
In case you didn't know: In the US you tip people who serve you in restaurants a substantial percentage. Don't think you can get away without complying with this medieval process, because the 'waitperson' will probably chase you down the street with a question about what they've done wrong.
The wages of waiters and waitresses are usually only a few dollars per hour, so they survive on your tips. Today's rates vary between 15% in rural areas to over 20% in the City. US income tax assumes that servers bring in at least 8% and will probably investigate if they claim less than 10% average.
Checking the income is done by means of the bills, so leaving cash on the table is better than to include it in the credit card bill. From the tipping fee, the servers sometimes have to pay the bartenders, table cleaners, dishwashers, and other peripheral functions to maintain a high service level.
In case you found the food horrible, you have to refer to the cook or the manager for this, not to the poor waitress that served it.
For Europeans, being used to all-in wages, the hassle of calculating the tip and rounding the figure to the nearest dollar increment is a never ending struggle. Americans do not know about the easier alternative and accept it as a part of life. They are eagerly awaited in many European restaurants as they tip according to their country custom.
Think about this intricate micro-cosmos of social dependencies next time when the waitress presents you the bill. Surprisingly, it is not common to tip people who fill up your automobile at gas stations. It used to be common to do that, but since they do not wipe your windows or check your tires anymore, tipping is also not done. By the way, you're not allowed to fill up your tank yourself in the State of New Jersey, and the tank has to face the pump.