If it's a company's sole method for determining, "do I want to reach out to this person," then, at best, it's short-sited. I mean, at the very least, trolling GitHub to find people to cold-call is likely to have a low conversion-ratio: 1) you're assuming that the people you're cold-calling are going to be receptive to such a call; 2) you're probably not the first, at this point in the recruiting game to have contacted the person this way.
As to the fairness of trolling GitHub while not contacting those who don't have a GitHub presence? It's like my parents told me from a very, very early age: "life's not fair."
Personally, as someone who's looking for more talent to add to my teams, looking at GitHub profiles is simply one path to getting to understand someone's skills. Personally, I'd rather see someone's blogs - as they tend to provide a better view into how someone solves problems and their personality - than only looking at bare code. Frankly, it's at least as much the softer skills - "how people think" and "how people communicate" - as it is the technical stuff that makes me interested in a candidate. If someone's a great tech but a horrible communicator, they aren't going to last long in my organization (or, worse, they'll cause others to leave)
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