Also to write free software that is recognized as useful, is a major hint of a possibly good programmer: it means that he is passionate, and is able to identify a problem an write and design a good solution. And there is a meritocratic salt in this soup, as open source can be considered, in some way, a way to show that you as a programmer are worth of consideration.
So I admire github for this hiring strategy.
Edit: is just my feeling or I'm feeling some open source hostility in this thread?