Agree with you, the percentage for the tips in US are:

breakfast and lunch are 15%, dinner is 20%, For carry out, if the waiter take the food to your car, should pay 10% tip. I always follow these rules

However, the reason for racial discrimination is complicate, sometimes, there is no reason. People just against you, or look down at you only because you were born in certain country, happen to be in certain race.