They did this because the new keyboard is bigger.
Pretty sure that's the recommended choice now. Saw it in one of the WWDC videos, landscape navbar on iPhone is "compact" mode, while in landscape it's regular.
It's the new behavior of the OS.
IMO it will behave this way just in screens smaller than the iPhone 6 one.
I'm guessing they're using the same "Immersive Full-Screen Mode" that Android introduced in 4.4 KitKat.
I noticed this behavior in many of the system apps. Do you think this will happen maybe just in screens smaller than iPhone 6 because of a lack of space?