The problem here is not so much about open vs closed-source. GitHub is a large centralized social network controlled by corporation, and we should never expect a corporation to behave ethically because ethics too often contradict with commercial interests. There is so much possibility for abuse, they do censorship and there are ways in which they can discriminate against certain projects and licenses and promote others. Even if Microsoft or previous owners have not done anything bad with GitHub yet, it does not mean it is acceptable to give so much control and influence over the open-source community to a single commercial company.