Apparently, HBO's Game of Thrones, in need of a large number of heads for a head-on-spikes scene, used whatever was available, one of which was a head of George W. Bush. The showrunners deny that there was a political motivation. They just needed heads and took what they could get.
However, there is a subtle political message here. All the other heads on display were "good guys" who had been beheaded by a psychotic teenager. So the implication is that George Bush is one of the "good guy" Starks and his enemies are psychotic Lannisters.
Which is why I think the showrunners were telling the flat truth.