United 93 will make people angry with terrorists, and make them more inclined to fight our nation's enemies. This does not serve any useful purpose, if the ultimate goal is to tear down President Bush.
Farenheit 911, OTOH, will make people angry with the President and make them more inclined to ignore our nation's enemies. This was very useful, because it eliminated any distractions that might be caused by a productive war on terror, and focused attention on what was really important, which was electing John F. Kerry in 2004.
You just gotta look at these things the right way...
Fixed that for you :)