There are far to many posts for me to read all of them, but I have some stuff to add.
1) It absolutely sucks that most American tourists and sadly americans in general give that impression to the rest of the world. It's seems most countries believe we are arrogant, loud, obnoxious, uncultured, and self absorbed. Sadly not many have done much to prove otherwise.
2) If you KNOW you work in a high tourist area, why don't you simply ask what type of currency they will be paying with before hand? Would save you both a headache, and you wouldn't get accused of anything byt he americans.
3) Have some slight patience, like any job working with people you are gonna meet 1 great person, 8 normals, and 1 jackass out of every 10 people. Unfortunately America is still in its infancy culture wise and the school system and media teach people that we are THE superpower and that gets kind of ground into from day 1. the culture will evolve over time I'm sure, but I assure not every american is that way. Usually just the ones 30+. This generation seems alot more aware.
4) This is just more of a gripe/observation. America is a melting of all cultures. America tries to cater to all immigrants. Period. They get better health care than citizens, free money, help with housing. From an american stand point I think that people think other countries should bend to help us and make us comfortable in the same way. Perhaps they are slightly resentful of all the help and catering given to all the cultures in the country and dont understand it when it isn't given back out of the country.
The only thing I can promise you is that there will always be idiots, always be assholes, and that the only thing to get you through it will be patients and understanding. Good luck, americas culture is still young, and a large majority arent taught about the rest of the world. @ 16 I'd hope you wouldn't start developing a prejudice already ;p