When you are just starting out and have no real leverage in the deal, sure. But it sounds like the GitHub guys were the ones holding all the cards in this deal, maybe they made sure the terms of the deal excluded that "general rule".
There is no way a VC would invest if there weren't SOME exit in the future, or at least a convincing story (independent of veracity) told to make it seem that you're open to an exit.
The exit could be M&A (although it would have to be $3-5b to be on-target, and in that range, you need to be able to IPO as a credible alternative when negotiating the M&A).
