I don't think it's just Americans, I'm sure there are people in other countries thoughtless about their trash.
That being said, it does seem like smokers who would not consider themselves litterers don't always think twice about crushing their butts on the ground and leaving them.
I even strongly encouraged my husband in my gentle, wifely way to not throw his butts over the side when we were sailing. He started putting a little water in a soda bottle and disposing of them in there. Disgusting, but at least the butts disposed someplace more appropriate than the water.
Sooo happy he quit smoking....