I think the answer goes back to my above post, about being "A Little bit MIHOP". I propose that many Truthers, consciously or not, believe that the events of 9/11 can't be "A little bit Natural". Either they were completely natural (a possibility they discount automatically), or they were completely unnatural.
Why must it be completely unnatural? Well, we've seen the answer to that right here: the "first time in history" argument. Were any Truthers to admit the possibility that WTC 7 could have collapsed due sole to the damage and fires, they would have to accept the possibility that the same happened in the case of the Twin Towers. Were they to simply ignore WTC 7, or allow as to how it fell without assistance of the Conspiracy, that would open a hole in their argument that anyone with any brains at all could point out.
Thus, in their propaganda war, they must insist on the collapse of WTC 7 being a deliberate action on the part of the conspiracy, and the fact that that makes absolutely no sense will continue to be swept under the rug by our local truthers.