If they released the movie and anything happened to movie goers their companies would be destroyed by the lawsuits. My guess is that if the US government instituted a "terrorist exemption" from lawsuits they would have released/shown the films. Simultaneously, the government doesn't want to create an exemption because they want companies to do everything possible to protect themselves from attack and an exemption would create a disincentive to do so.

Classic catch 22.