If you believe in open source so much, why is Github a closed-source product?
Many businesses, particularly SaaS businesses, have managed to be hugely profitable while making their code open source, so I don't accept the argument there.
And considering that the choices of many open source projects to host there and standardize on the Github ecosystem were a big factor in enabling GH to become what they are today, at some point I would feel an obligation.
It's their business and they can run it how they see fit, but it's something that has always bothered me.