This to me does make sense... Kinda.
When the oil cap is on the engine is constantly creating a vacuum, which could be like saying the engine is "pulling itself", which creates a constant vacuum. You may notice on some high-powered Chevy motors that the valve covers have breathers (usually have K&N style filters on top of them.) The picture you provided with the filter that replaces the oil cap... I just don't like that idea, plus the design scares the crap out of me. (What is holding it on?!) Air filters block most of the airborne debris, but never 100% so who knows what is getting mixed up with your oil.
Anyways, removing the oil cap eliminates the vacuum in the crankcase which in turn allows the engine to move more freely. It would be like going from a shirt that is a little snug to one that is a size or two bigger.