The key ingredient in the 'negative' fallacy is the lack of time dependency. The fallacy does not address a specific point in time - i.e. at this moment, the switch is NOT ON. Instead, it offers the premise of proving something that never existed - in this case proving that the switch was turned off. Looking at the switch in the ON position, there is no way to prove that it was ever off. Thus, the fallacy.
The earlier fallacy brought up was this:
I'd be shocked if someone could prove that UVA didn't help organize and pay for the anti-protest gathering in addition to inviting them to town.
How can you prove that someone did not do something? How can you prove that Suppressed did not turn the switch ON regardless of whether the switch is currently ON or OFF? Sure, I saw Joe turn the switch ON. But what about before that? Could Suppressed have turned it on then? Just because no one saw a tree fall in the forest does not prove it didn't fall.