It is inevitable that facts would come to light, facts that Bush and cronies wants left in the dark.
In a real trial, the deep financial connections between the Bush and Bin Ladin families would be brought out in the open. The statements that Bush told the FBI to back off investigating the Bin Ladins before Sept 11th, and that Osama could easily have been extradited or captured by the US over the past 5 years and the troubling question of why US jets did not scramble and that the Patriot Act appears to have been written before Sept 11th would all get into the public eye in a real trial.
This is why Bush Ladin needs secret trials where everything can be controlled and the puppets who carried out the plan can be safely silenced.
Text on Military Tribunals