This has been discussed on debian-legal :
http://lists.debian.org/debian-legal...4/threads.html ( scroll down to see the thread )

Knoppix complies with (b) hense the written offer to provide sources included on the CD.

This is becasue the sources for 1 CD is 3 CDs worth of source code and too much for the mirrors. However, All the sourcecode is downloadable from http://www.knopper.net/download/knoppix/ for knoppix-specific stuff, or from the debian mirrors.