Becoming a member means sharing your personal information with GitHub and abiding by their terms of service. It also implies that they will then track and profile locations where you sign in, technologies you use to interact with their services etc. That's a lot to give for "I want to browse a piece of code someone made for free".
Facebook recently estimated that knowing about a user costs close to €11/month. That's a lot of money for hosting and indexing git repositories... I guess we need an alternative.