I don't see how this is a good policy. By happy accident of languages, we have no US-specific Wikipedia, in which Americans get to decide how their Wikipedia is going to look---they have to share their English Wikipedia with the opinions of all other English speakers on the planet who choose to participate.
Ermmmmm even if their were a US-english specific wikipedia, still everyone who considers themself to speak US-english would want to participate. I do not see the difference. Also on nl we welcome everyone who speaks Dutch. Whether they happen to be Belgium/Dutch/Surinamese or not. For all I care a Dutch speaking Martian contributes. I mean we have Dutch speaking trolls so why not :)
Walter/Waerth