I'm pretty sure they do it because the keyboard can't source the required 500mA of current. They "proprietarize" it in order to keep people from plugging high-current devices in.
The keyboard itself contains a 2 port USB hub (regular ports, btw, with enough juice to power a Wacom tablet and charge my phone at the same time), so I'm not sure that makes sense.
Never mind, then. I have no idea why they did it but there's probably a perfectly valid technical explanation.