Personally, I've made far more open source contributions at my current (awesome) job than I ever did while on the job market. I use GitHub because it makes coding more fun, and because I get a huge thrill every time someone tells me they used one of my projects.
I also don't understand the OP's hostility to the open source movement. If you don't want to share your code, then don't share it! That's totally fine. But I'm very conscious of the fact that almost all the tools I use every day -- Linux, tmux, vim, Ruby, Python, etc, not to mention hundreds of libraries and plugins -- were all developed by people in the open source community. Often these people were working for free or for far below-market wages. Whether or not OP likes the OSS movement, modern software development is built on open source tools. Personally, I feel an obligation to try to give something back, even if it's something as small as a REST API wrapper.