Page 59 of the USB HID Spec [1] is the offender, it states:

   The following table represents the keyboard input report (8 bytes). 
 
   Byte Description 
   0 Modifier keys 
   1 Reserved 
   2 Keycode 1 
   3 Keycode 2 
   4 Keycode 3 
   5 Keycode 4 
   6 Keycode 5 
   7 Keycode 6 

Which is where the 6 key roll over comes from. However this is really the minimum implementation so that BIOS'es can use USB keyboards. If you read the spec carefully there isn't anything that says you can't implement a different reporting structure, as long as your HID driver is expecting it. My guess is rather than ship "special" keyboard drivers they just use the standard one, and they opt to use PS/2 connectors which work in the same way (they are really just a serial port and can send as many keys as they want) for the 'corner' case of full n-key rollover.

[1] http://www.usb.org/developers/devclass_docs/HID1_11.pdf

Thank you. So the issue is that Das can't be bothered with coding and maintaining custom drivers for every OS under the sun. I guess that's fair.

Also, I've never actually run into a problem using it over USB on a Mac or PC, so I guess it can't be that bad! (I do play games, but this keyboard goes on my work machine in whichever office that is.)

You don't even need to code custom drivers though, you can just present the keyboard as multiple keyboard devices to the host.