I wonder if GitHub couldn't clear this up by picking a fairly restrictive license (maybe good for personal use, but not for commercial) and make that the default (as opposed to no license, or a very vague one as we see now).
Then if I wanted things looser, I could pick from other licenses, if I wanted things tighter, I could pay the money for the private repositories (again, this is what I do).
The problem is that to make things private costs money...not a LOT of money, but some. But it occurs to me that the intent of making you allow people to download and fork would be to make your code available, not necessarily to enrich the downloaders, but in the spirit of knowledge sharing.
So a default "personal use" license, but no commercial use required on the free GitHub seems like a reasonable compromise...and more explicit than the current situation.
I doubt they'll be calling to ask my opinion, but there it is.