Yup. You're not restricted to the servers you play on, but if I'd wanted to play on a US server I would have needed to buy a US copy of the game. Same deal currently with WAR.
In addition, there are a good number of American's abroad who play from other countries. Not to mention our servicepeople who buy the American retail version.
Plus, the Australians don't even have their own realm servers. They can play on American-Oceanic servers, which Americans are free to play on as well. That is, of course, if they choose to play there and not on the usual American realm server.