I had the same misunderstanding as well. However, I think they ought keep Windows 10 Insider program available to everyone free of cost and without having to pay for a license. I feel like this will fall on deaf ears though. Microsoft watchers say Windows already treats stable as a testbed for the enterprise users who withhold updates for n days. So, a new update shows up without much testing, breaks a bunch of stuff on consumer hardware, Microsoft finds out thanks to logs or twitter chatter and fixes it, and everyone including enterprise hardware take the updates.

I think eventually Windows being source available is possible but I doubt it will be free in a meaningful way.