It bugs you because you don't get get it, freely stereotype and readily tell people how to spend their free time.

I work for a bigco that doesn't contribute to open source. I used to work for a bigco that was one of the biggest contributors to open source. I spend spare time working on an reasonably sized open source project, of which I was one of the founders, hosted on, yes, github. I have interesting (to me, atleast) side projects, mostly hardhacks, which I don't necessarily build to share. I do it for fun. Obviously, I read HN.

I never require open source experience when I hire, because my experience in both the 'closed' and 'open' worlds tells me that great scientists and engineers exist in both worlds and using a VCS account as a filter is crude. This is just another facet.