The reason you see many contests/giveaways being limited to U.S.-only is because the laws and regulations vary from country to country. Even in the U.S., laws vary from state to state. Trying to make sure that the contest doesn't run afoul of laws and regulations throughout the world is, to say the least, challenging. In some places, certain types of contests/giveaways are illegal.
With so many of these things, those conducting the contests would love to be able to make it open to everyone; but, for the aforementioned reasons, they often are not.
I am going to remove a lot of off-topic discussion from this thread. This post should answer the question about why it is limited to U.S. only (as are many such contests/giveaways by countless other companies).