It's abnormal behaviour. There's no reason to hang your country's flag in front of your house. I mean... maybe if you live near the Canadian or Mexican border and are feeling particularly patriotic or insecure.

> I’ve seen people from all over the world wear or display their own country’s flags.

You may have imagined it. If not, could you say where? It is a uniquely American thing. I mean, Indians will pull out a flag quite often, but they're perpetually at war with their neighbours so it's kinda understandable. In the US you'll see flags on houses in the middle of buttfuck nowhere, owned by people who've never met another foreigner. It really is bizarre behaviour.