OK, well, only one statement from my whole post has been used to make a point, which everyone does and that's fine. Just wanted to remind you guys if you read my first response to this situation, I pointed out that people are rude anywhere and everywhere. Sure, when I originally went to London, I was pretty surprised that none of the strapping young men on the train would offer their seat to an old blind lady, where I thought to myself in Texas that would be common courtesy. And I certainly didn't chastise anyone for not standing up for her!
Then again, I was also reflecting on the fact that in college in Texas, I watched people literally shut the door in a disabled person's face while he tried to manouver his crutches and open it for himself. So maybe those Texas manners weren't really as courteous as I remember them being at times. Poor customer service happens anywhere. I understand what you are saying about people getting a bad opinion of Americans for telling someone how to do their job, etc., but I don't think Americans should just always walk around and keep their mouths shut for fear of giving a bad impression of the country itself.
When we were in America, DH was shocked at the complete lack of help by salespeople, and he made a few snippy comments to them himself, and he is a pretty shy guy in public, but just felt he didn't have to tolerate being treated that way. I doubt that left those salespeople thinking all English people are rude, and if they did, so what? Why does everyone care so much what other people think? What's wrong with speaking your mind sometimes if you think something warrants it?
Only the OP was in the situation, and she did what she felt was right. I wasn't there, so I can't say how serious an offence the situation was, but why should she keep her mouth shut just because she is American? No one is saying its great to "make a scene" but why just walk around with your head down because you are afraid of what others might think?
If I give a few people in a store a reason to bash Americans, so what? There are maybe 50 people here I have had conversations with who thought I was canny and nice and whatever and maybe I improved their opinions.
Sorry to rant, but really, it gets old that people are always so worried about someone giving a negative impression of Americans. Each person should decide for themselves whether their behaviour is appropriate or not, American, English, or whoever.