No, I don't understand that either. I think there's potential for insistent illogicality on both sides of the debate.
People who dismiss the theory that 9/11 was an inside job as obviously ridiculous, but then don't have any explanation about legitimate questions such as how a passport survived unscathed and identifiably in a fire that did immense damage to an aeroplane; why was the complete collapse of Building 7 reported by the official news channels when it could still be seen behind them standing before suddenly collapsing 20 minutes later; how was the Patriot Act all ready to go in just six weeks - and not even just any six weeks, but six weeks after all of that had just happened? Also the reports that the Bushes and Bin Laden's close family members had met socially shortly before the event; and the claimed testimonies from very experienced military pilots that they couldn't have flown into the buildings with such accuracy etc.
As PP said, whether or not you accept the veracity and evidence or not of these questions/conclusions of something afoot is one thing; but just instantly huffing and harumphing that it's so clearly all ridiculous, and that anybody asking these questions must be mentally ill, doesn't necessarily put you in a great intellectual light either.
On the other side, the people who doggedly insist that Finland doesn't exist; birds are actually robots that recharge by sitting on telegraph lines; the royal family are actually, physically reptiles and not human at all - because they read it online and so it must be true... These can all be very easily gainsaid with very simple logic, and none of the claimed 'evidence' can be objectively said to merit any serious consideration.
Oddly enough, even the most staunch automatic deniers of all conspiracies in the west never seem to have much difficulty in assuming the exact opposite when it comes to allegations of what the governments of Russia, Iran, North Korea and China have been up to. Unless you genuinely believe that all/certain governments would never, ever lie to their people about anything, it does seem to me to represent a bright, open mind if you critically examine the evidence and the merits of the suggestion. Insisting that it MUST be true, or similarly that it CANNOT be true, without any consideration at all of serious, at least possible objections or questions just seems rather intellectually dishonest to me.