Part of this may be explained by higher load factors in the US. In other countries where the airlines' yield management systems are not as sophisticated, or the airlines operate as money-losing government monopolies with lots of empty seats, this isn't likely to become much of an issue.

I have seen seat poaching in other countries, but it's usually quickly resolved between the parties. I have never seen anyone in another country approach a FA about it. Perhaps approaching an authority figure rather than dealing with it directly is a more American attitude due to the more individualistic culture.