It's a system which disproportionately rewards the lucky few whose employers allow them to either open source their work, or contribute to open source projects... or the people who spend their free time coding. I know plenty of folk who are damn good at their job but like to spend their free time on other, unrelated hobbies.
I'm actually lucky enough to fall into the former category, but I feel that if you only hire people with a respectable presence on github, you're disregarding a large number of people for something beyond their control that doesn't accurately reflect their ability.
>you're disregarding a large number of people for something beyond their control
What you spend your free time on is not beyond your control (at least when you're a software dev and presumably paid enough to actually have free time). And you might decide to spend it on other, unrelated hobbies. That's cool, but you should be aware that unless you can present that hobby as interesting, unless you can sell it as something that helps you grow, you are making yourself look less good that someone doing the same kind of work, and then doing OSS in their free time.
A company is not going to disregard you for not having a respectable github presence if no other candidates have a github presence either. But if there's a clear disparity, why should they take an extra risk by hiring you, rather than hire the person that can show them their actual contributions?
"at least when you're a software dev and presumably paid enough to actually have free time"
...or you know, don't have family obligations.
I do agree to some extent with what you're saying, but as others have stated, it should be one of many factors taken into consideration, and it depends on the culture the company is trying to foster.
I personally have looked at github profiles of people I'm interviewing, if they chose to share it with me.
Choosing a company to work for is not beyond one's control. It is possible to specifically look for a job that allows contributing to open-source in some way, or, in other words, to blacklist those who explicitly forbids that. I've done this in both two cases I was on a job market and still enjoy this decision.
Maybe I see the world through rose-colored glasses, but I don't see why sane employer would like to forbid contributing to open-source project if e.g. this project is used for work and has some bug. And there are always bugs or inconveniences if you use something heavily. For some industries it is even hard for a developer not to be on Github, because a lot of software she uses is on github, and in a lot of cases the best way to fix the issue she's working on is to contribute to open-source project that caused this issue, instead of e.g. working around it by some hack.
I agree that discarding people without respectable github presence is bad (unless you're looking for a very specific kind of people, e.g. for marketing purposes). But I can see how not being on Github at all (even with minimal presence) could be a bad sign in some cases. Of course, industries are very different and this shouldn't be a general rule.
I'm actually lucky enough to fall into the former category, but I feel that if you only hire people with a respectable presence on github, you're disregarding a large number of people for something beyond their control that doesn't accurately reflect their ability.