Ok, so it's not only Americans who wear shorts outside even when it is cold, but it is definitely one of the most obvious signs that someone is a tourist. Of course, cold is subjective. In some countries, it may be in the upper 60s, and the locals will think it is cold, while tourists (especially from America) may think it is quite hot. That is when you'll see the locals bundled up and the Americans in shorts.
It is pretty common to see Americans wearing shorts outside, even if it is snowing or there is snow on the ground. Aren't their legs cold?