Ok, a little history... at this point in time, from a purely functional standpoint it doesn't matter in the least whether it's Italian threaded or English threaded since the ball bearings themselves are never running against the actual surfaces of the cups. But back in the day (yes, I was alive then), and before cartridge Bottom Brackets were prevalent (some of you are probably asking what those are even), the bottom bracket cups screwed directly into the frame, and the ball bearings were loose and rolled against the inner surface of the cup and the axle itself. The directional forces of the balls against the cups would, through the process of precession, act to turn the cups in counterclockwise direction (the normal direction to loosen things). If the bottom bracket cups weren't installed very very tightly, that was enough to often loosen and unthread the entire bottom bracket cup. It happened to me while cycling across the southern US one year on my Basso, and I didn't have a BB tool to keep it tight enough, so for a while, till I got to a major town with a tool to tighten it properly it was a royal pain in the ass. This was not at all uncommon. So, to combat this, the left hand threaded bottom bracket on the drive side (Enlgish) solved this problem completely. Now, if the BB wasn't exactly tight enough, the process of precession would act to tighten it, and not loosen it.
Then came the cartridge bearing Bottom Brackets which were completely self contained and just screwed into the threaded frame shell. The cups were there just to hold the cartridge but the ball bearings themselves never actually spun against them and were contained unto themselves. In fact, with the cartiridge type BB's, it didn't even matter whether or not you faced the frame shell, so long as the threads on both sides of the frame were aligned.
Fast forward to today and there really is absolutely no reason to be using an Italian threaded BB shell, unless you really want to just be different than what the standard has become (at least as far as threaded shells are concerned). Actually, there are no real BB standards these days, but that's another topic.
So why Pinarello still uses the Italian threads is beyond me. It doesn't matter, as the precession process is not relevant with todays BB designs, so functionally it doesn't present the issues that it did in the past, but still.... I guess they just want to hold onto a little bit of their "Italian" heritage, even though it's probably the worst aspect of the Italian heritage they could possibly want to hold onto.