Charlie Brooker put it better than I could:
Like nutbag creationists, they fixate on thinly spread, cherry-picked nuggets of "evidence" and ignore the thundering mass of data pointing the other way.
And when repeatedly pressed on that one, basic, overall point - that a conspiracy this huge would be impossible to pull off - they huff and whine and claim that unless you've sat through every nanosecond of Loose Change (the conspiracy flick du jour) and personally refuted every one of its carefully spun "findings" before their very eyes, using a spirit level and calculator, you have no right to an opinion on the subject.
Oh yeah? So if my four-year-old nephew tells me there's a magic leprechaun in the garden I have to spend a week meticulously peering underneath each individual blade of grass before I can tell him he's wrong, do I?
Wonder if he came up with that line himself? R.I.P.
