Maybe the producers are making political points ?
http://io9.com/5918114/george-w-bushs-decapitated-head-appeared-on-game-of-thrones
http://www.theblaze.com/stories/george-w-bushs-head-appears-on-game-of-thrones/
The 2nd URL link points out that the book is written in 1996, and the producers related their show to US presidential administrations that did not yet exist at publishing time.