There are two reasons to wear a beret.
1. To be French.
2. To disguise yourself so you can appear to be harmlessly French until you catch a Nazi and slit his throat.
American special forces get to wear berets in memory of the second group.
The American Olympians should not be wearing berets unless they are planning on marching right behind the German team...
Usually the opening and closing ceremony clothes are designed to highlight something of the culture of the country. It is tough to do that with a country as large and diverse as the US, but Ralph Lauren's French Yacht Club look is not part of our society in any way (unless you are John Kerry).