Do you have a source for this? I'd be very disappointed to hear that if it were true.
EDIT: It appears that this isn't true. From a GitHub employee in freenode/##atom:
<jonrohan> EvanDotPro: it will be open source, and other platforms, when it's out of beta
<chance> jonrohan: where are you getting that information?
<jonrohan> chance: i work at github
> Atom won't be closed source, but it won't be open source either. It will be somewhere inbetween, making it easy for us to charge for Atom while still making the source available under a restrictive license so you can see how everything works. We haven't finalized exactly how this will work yet. We will have full details ready for the official launch.
Either jonrohan is misinformed, or he's using a rather tortured definition of "open source" - even the Open Source Initiative would disagree with that usage[0].
I must say, I'm a bit disappointed by the fact that Github doesn't seem to be on the same page internally about this; it's a rather important piece of information.
EDIT: It appears that this isn't true. From a GitHub employee in freenode/##atom: