When I was younger I tended to walk that way, but I'd get people yelling stuff out their car windows and throwing things (like monkeys throwing poop), but I somehow figured out that standing straighter (eye line parallel to the ground) would help.
Thirty years later, I don't worry about it much, but I think people are too intimidated to mess with a 6'/195lb. 40 years old male person in such ways. Either that, or I'm completely off their radar.
I've wondered if it's a matter of reducing sensory overload. When you look at your feet the view is pretty 2-dimensional and simple, whereas when you look up there's depth involved and a lot more objects to take in.