In the past I have said that people who object to my questioning the accepted story regarding 9/11 have a Thar Be Dragons mentality.

You ask why there's no released video of flight 77 striking the Pentagon. Do you really expect me to believe that George Bush personally ordered the attack?!

More formally this fallacy is known as the Appeal to consequences.

Appeal to consequences, also known as argumentum ad consequentiam (Latin: argument to the consequences), is an argument that concludes a premise (typically a belief) to be either true or false based on whether the premise leads to desirable or undesirable consequences. This is based on an appeal to emotion and is considered to be a form of logical fallacy, since the appeal of a consequence does not address the truth value of the premise. Moreover, in categorizing consequences as either desirable or undesirable, such arguments inherently contain subjective points of view.
--Wikipedia