ATI publishes their specs so open source people write their own drivers (ala the new X 4.3).

Nvidia does have a "Linux Exception" clause to their licensing agreement (see their website) that allows for the distribution of their drivers. Consequently several distros DO include them. I think Xandros does and I know for sure that Morphix has them on the Gaming Edition.

I think the reason why the Nvidia drivers have not appeared on Knoppix is because there are no programs on the CD that won't work fine with the standard nv drivers. The only reason why I can think of that they need to be included on the CD is for HD install purposes.