Look, I work on a lot of proprietary stuff as well, but even if Github didn't exist, we'd both be subject to the same "unfairness." Here's a job interview, circa 1981:

Candidate 1: "I brought this fanfold printout of the source code for _____ I wrote with me, have a look.

Candidate 2: "I can't show you any of the code, but trust me, it's good."

Github merely drags that conversation forward from 1981 up to 2011. When you and I choose to write code that is locked away in somebody else's vault, we ought to charge extra to compensate for the fact that we might as well have been surfing Oahu.