This is purely speculation, but if you are an insider and have come from a genuine copy of 7 or 8 then Microsoft already knows that you're good for it. Otherwise the same argument might apply to someone who reverted to 8.1 12 months from now, for whatever reason, maybe accidentally when they just wanted to do a refresh, and were then told that they'd have to then go and buy 10. As I say, this is just my opinion, but if you have upgraded to the official release already, then you now own it, and if you reinstall a while later having left the program, MS still have the information on your account confirming you claimed your upgrade, and will then just verify against the Key on your machine that you can install it.
We might still be insiders, but we're also now legitimate owners of windows 10 just like everyone else. It does feel rather odd though not to have a physical key and disk in my hand. I feel a bit naked