Tips: fair or unfair?

One thing I did not like in American restaurants: giving tips. Ok, I am not a mean person, I just was not used to it. The only situation in which we give tips in Brazil is when we park our cars in the street (not a parking place), and we give some money for those guys who offer to keep guard over the car or the motorcycle. So, we explicitly agree in giving tips.
In US you have to give it everywhere you go, except for some fast foods.
Waiters do not earn a full payment as in Brazil, so most of their money comes from tips. And everybody knows it is not mandatory, but you feel bad when you do not give it because it seems that you are not giving the waiter/waitress the money they deserve. A good consequence of tipping is that all of them are soooo polite, nice and talkative!
They do their best to make you feel good and satisfied and, as a consequence, give them good tips. And some people say that if you do not give tips in the first time you go to a restaurant, the next time you go there the same waiter may not be so good to you. The normal wage for the tip is 15% of the final bill, but if the waiter is really good, some people give 20%. Once an American friend posted in her Facebook that she had earned a 60 dollars tip. And I was like WHAAAAAAAATTTTT???!!! What did this girl has to do? What I have learned with this experience is that if I am helping those people who really work hard standing up for long hours and serving people being as much nice as they can, why not rewarding them for a good level or service? Some of them are still in college, can you see how they have to work hard in dealing with classes,  homeworks and the restaurant hours?! These guys definitely should be rewarded.