Rude Americans - Spoiling It for Everyone

I'm usually the first to say that I find Americans to be a friendly, polite bunch in general. I live with a few of them and believe I've seen enough Americans over the last few decades to be able to opine on the subject.
The stereotype of the loud, demanding American however, was well represented when I flew back from Heathrow airport last week. What on earth happens to perfectly decent people when they are traveling outside their country and, presumably, their comfort zone? Why do they expect people from other countries and cultures to behave as if they're from the USA? (in this case - I'm well aware that there are ugly British tourists all over the place too.)
The incident wasn't actually that horrific but I don't think the Americans in question even realized that everyone else was aghast at their tone and attitude.
I returned my car to one of the rental places, and as usual, there was a bit of a wait to get the car checked in and be on my way. This time they said they were short-staffed. Obviously, since we were at an airport and there were flights to be caught, the atmosphere among some customers was a little tense. One older American couple in particular were getting more and more irritated, and matters weren't improved when the employee checking everyone through seemed to abandon them and disappear into the office. (Apparently the couple had amassed so many speeding tickets that it had to be handled by the manager.) When the employee came back out with the manager, she (the manager) went straight over to the Americans.
Apparently manager attention wasn't good enough for them and the American woman proceeded to shout "Excuse me" very loudly and in the general direction of the first employee, who was now attending to me. Even when the female manager explained that she would now be handling the situation, the couple continued to demand that the first guy come back and apologize (or something). Never mind that everyone had flights to catch and said apology would slow things down considerably.
Unfortunately, when I was at the desk, signing papers, the Americans came in and proceeded to demand my guy's name. Intimidation and name-taking is a tried and tested method in the USA but this guy didn't bat an eyelid, merely smiling and saying they didn't give out personal information. American couple then demanded to see the manager (????), who was now sitting right in front of them, checking them out. Their complaint? The male employee was being rude. The manager quietly replied "He's not being rude" and, to my astonishment, the Americans both proceeded to jab their fingers in the general direction of the bemused male employee and shout "You're rude, You're rude" at him.
I've lived in the US for 23 years and I can confidently say that jabbing your finger and insulting people is not considered good manners in any part of this country. What were they thinking?
As you can imagine, I almost had to clamp my hand over my own mouth to stop myself from saying anything. (The only thing that did stop me was the inevitability of them being seated right next to me for the 8 hour flight back.)
What would you have said, if anything?