Though much of GitHub is open-sourced, the core app itself is not (yet). We just don't want to not be able to use the winning entry to best improve the site fo

Though much of GitHub is open-sourced, the core app itself is not (yet). We just don't want to not be able to use the winning entry to best improve the site for the 150,000 open source projects we provide free hosting for because the winning code is GPLd or something. We are trying to encourage the development of open source solutions to this problem so everyone can benefit, though - all entries have to be open source at the end of the contest, that is a rule. We want everyone to learn and be able to use the results of this contest. I would encourage the use of more permissive licenses, but I didn't want to enforce it, thus the disclaimer exempting us. Otherwise the 100,000 users we provide free Git hosting for don't get to benefit from the best solution, either.