Really? Apple says, "And three downstream USB-C ports (5 Gbps) offer additional connectivity and power to compatible devices and accessories." What makes mice and keyboards incompatible?

Edit: according to this random internet stranger, it works fine: http://www.apple.com/shop/question/answers/product/HKN62LL/A...

I don't know where you got that impression. I have this LG 5K display and I'm typing on a keyboard connected to it right now...

Untrue. I have one and works just fine with my mouse, keyboard, sdcard reader, external hard drives, etc.