I applaud this decision by Github, as it allows expression to be expanded without fear. I do understand the concern that an employer could have (as I did run a few small businesses), but hacking out a few lines of code while at work during a break, for Open Source projects shouldn't be claimed as corporate IP.
This is a thing of opinion depending on the reader, but I digress. I am totally in favour.
What's mine is mine.
In other news, it is critically important to actually read the documents you sign when you start on with a new employer.