Why has the U.S. Postal Service blocked me from buying books from the United States?

Nasir Khan, June 3, 2014

Last month I ordered a book from Oakwood Books through Amazon.com (USA). Today I was informed by the bookseller that the book package has been returned to them by U.S. Postal Service marked “Sender or recipient name matches a denied party/entity who may not send or receive exports.”

I wonder what is the reason for putting this ban on me. I have never done anything wrong against any American or anyone else. As an academic I buy the books I need from different countries. Can anyone explain this strange decision on the part of the USPS?