Look, this is nice for github employees, but it's worth noting that this is not an easy decision for all companies to make.
We (developers) aren't like factory workers cranking out widgets by the hour. We're paid to think deeply about challenging problems. For me, that means chewing on a problem for a long time, often when I'm not at work. Honestly, when a problem is challenging, I can't get it out of my mind. I'm thinking about it, at least a little bit, on the weekends -- heck, maybe even while I'm sleeping. And those are the problems I enjoy the most.
When a savvy software company hires someone, they understand that this is how developers operate. They will offer handsome compensation because they expect this.
Ultimately, competition for your thinking time is a zero-sum game. If you're working on a hard problem at work, you might think about it at home. Similarly, if you're working on an interesting side project you might think about it at work. You can't help it.
That doesn't mean companies should never allow side projects at all. There should be room for negotiation. But if we don't acknowledge the basic facts of the situation, then we aren't negotiating in good faith.