Because there are people who do get accidental upshifts at the end of their cassettes. Even if not all the time. It's great that the system works great for you. Doesn't mean it will work great for everyone. Heck... I'm one of those crazy ones who don't like Campy. Sure, they look cool. But I just don't like having a different motion for upshifts and downshifts. I'd be a lot more likely to put SRAM on my bike if when at the end of the cassette, a requested downshift to the position of a normal downshift simply did nothing and didn't require further travel or pressure to avoid an upshift. But that's just me.