Dear Hung,
This is Alvin again with iTunes Store Support. Thank you for staying with me while we continue to investigate your issue today.
I have consulted a Senior Advisor on your behalf to explain the situation, and while Apple endeavors to provide the best quality products and services, I have been advised that you are not authorized to obtain content from the US iTunes Store. We are unable to provide further information regarding this situation. I apologize for any inconvenience this has caused.
For more information on Apple's requirements for using the iTunes Store, please refer to section B of the iTunes Store Terms and Conditions:
http://www.apple.com/legal/itunes/us/terms.html
I hope the information that I have provided proves to be useful.