I've lived in both Europe and the USA, both South and North (And I basically always wear a shirt while running, for the record). I think people don't realize that the US is generally just a lot hotter than most of Europe. Southern US is just freaking hot, majority of Europe is not. No shirt / shirt. Simple.
My own perspective from living in Prague - my main Czech training buddy would often run without a shirt, while I always wore a shirt.